My mother-in-law (of sound mind and still able to walk/cook etc.) has developed the habit of licking her fingers at the table after she eats. Up and down the fingers, not surreptitious at all. I find it nauseating, but I would merely look away if it were not that when she serves the dessert she has made, she touches it. Last night she put her licked hand flat down on the top of the brownies to cut them. I didn’t have dessert. Once, months ago, I screamed “Don’t touch it” when she was about to do it to the meat she was serving. She was kind of aghast and went and washed her hands once I had explained. But because I think it is a habit, I don’t think that saying anything will stop it. Is this correct? Should I just not eat dessert when I see her do it, or is there a way to politely let her know?
That’s gross. I have no solution other than don’t eat dessert, but yes, that is gross
That is just so bizarre, I would ask her, out of curiosity, why she does it. Maybe listening to her explain will enable you to have a brief conversation about why you (and probably others) never eat anything at her house after she does it. You probably won’t change her behavior, but your aversion to dessert will be out in the open and perhaps a reminder to her whenever you’re at her table.
Keep her away from any food you are about to serve. IOW…no trays of cookies or any other food on the table where she can reach. Perhaps you can pass trays of desserts for folks to choose.
Plate meals in the kitchen with the guests.
And put a bowl of Handi-Wipes next to her plate.
I doubt she would use them but gloves for serving?
It might be wise to let her know that people view that as unsanitary, and especially since COVID people are more aware than ever of things like this. Would your husband be willing to have the conversation with her?
D1 insists on everyone to wash their hands when they come in the house, after they play with toys or touch shoes, etc. I have to admit that I used to forget every once in a while, but she would always say, “make sure you wash your hands.” Now I have gotten into the habit of always washing my hands before give food to GD1 or touch GD2 (less than 2 months old).
I wonder if you could keep on reminding your MIL to wash her hands before she touches any good. I think it’s also a good idea of having wet wipes next to her plate, so whenever she starts to lick her fingers remind her to use the wipes instead.
LOL! No. After getting your responses, I decided to ask him to pay attention after the meal when she does it right in front of his face–so that at least I won’t feel so alone. When D24 is in town and eats with them, she notices the bad manners/lack of hygiene (my father-in-law coughs on the food). She doesn’t eat anything that’s been compromised. I honestly think it’s because my mother-in-law grew up on a farm, and my father-in-law grew up in a very working-class home. They used to get food on their faces regularly when they ate, and then I think that a couple they made friends with actually told them, because it gradually improved. My husband had some similar issues when we met (not the licking or food touching) and has improved over the years at my request. A marriage therapist told me years ago that it could help to think of their situation as coming from a foreign culture with different customs. But I am not going to eat the brownies! ever.
I’m kind of ocd about washing my hands when prepping and serving food so this would freak me out totally. I wouldn’t just be worrying about dessert but about what happened before that as well. At the very least can someone explain to her that if she’s sick she’ll make everyone else sick too?
Btw my husband grew up on a farm too and all of his family (5 siblings) are careful about washing up to prep and eat food. That’s not an excuse.
My family was working class growing up. No one licked their fingers at the table, and they certainly didn’t touch food when they did.
I think your husband needs to say something.
I’ll add…I don’t know if your MIL brings food to share at family meals, but if she does, I would suggest you give her prepackaged assignments (like unopened jars of pickles) rather than anything she cooks herself.
I would treat this with a little more compassion. She sounds like she is in good mind and body shape and so certainly you can have a conversation with her.
Put the shoe on the other foot. If this was you and a crazy habit how would you like someone to address it with you? I’m guessing you wouldn’t want anyone to present it as weird or wrong or as a sign of feebleness.
The wipes are a great idea. Use one yourself at the table to model and show acceptance for this routine.
I think I would (you or your husband) just say “Mom, i noticed you’ve developed a new habit of licking your fingers at the table! I’m wondering why you do that each meal.” See what she says. Maybe it’s something she does if she is normally eating alone (is she?).
I would also use the flu season or covid as a reason it would be good for all to use wipes or handwashing more often at mealtime. No one wants to get sick!
Also wanted to say that I wasn’t referencing OP NOT treating the situation with compassion - I was just saying even though it’s gross and bothersome keep in mind her situation as if it was yourself! 
Great advice.
So much better than just saying something is gross.
My mil grew up in a farm also. Her manners are pretty good but she has a habit of making a casserole in the morning and letting it sit on the counter to warm up for dinner.
I find this difficult but I haven’t died yet so I guess it’s not worth pointing out. If I did, I think I would have to address it and I think you should. Or ask your husband to if he would. Mine hates confrontation so it would most likely be mine to address.
My dad also grew up in a home that wasn’t very polished, some of his manners were rough. I did address them but he was my dad.
Offer to do the food serving
I wondered if she were now widowed and just doesn’t think about it if she normally eats alone.
My fil was a notorious double-dipper. No one would say ANYTHING to him. There would always be a spoon in the dip and plates available for serving yourself your own individual portion of dip - then one can double dip all they want!! But, he would never take a plate as he was always a grazer. I don’t mind grazers/grazing at all, but do NOT double dip!!
Her husband is alive–mostly disabled–can “walker” slowly around the house but resists a wheelchair so is housebound and she is limited because he wants her to stay with him(!). That’s why I have compassion. But THANK YOU to everyone who has posted. You’ve actually given me a much better understanding of how weird my situation is, and that I shouldn’t feel guilty about my feelings. It took her nine years to stop being nasty to me (I’ve known her for thirty), and she’s a huge control freak about food and the kitchen. To allow me to serve her food would be unpleasant for her. We never get to host because father-in-law can’t get up our stoop (four steps), so we bring stuff over to their house for our “turn.” The suggestion from @abasket is the most workable (that my husband address it, “I notice you’ve started . . .”). I’ll see if he’s willing, when I can work up the courage. As you can imagine, the entire situation is not ideal. Thanks again for all of your help.
First, this is super gross and I’m sorry. Second, your partner should deal with it.