It’s your sister. If you really think she’ll like it give it to her before the party. A day or more before. Then you won’t look like you either didn’t heed her wishes for the party or bought something because others brought gifts to the party.
Although I also think if one gives a gift despite “no gifts” one is accepting that they have no right to be offended if they never see the gift used and it ends up at a thrift store a month later.
One relative had a 50th anniversary party. They specified “no gifts”. They definitely meant it.
They knew that they were very fortunate in their 50 years together, and very fortunate in the success of the business that one of them started. However, I think that they also knew that they were near their end, and were already thinking about getting rid of the stuff that they already had. They did not need any more stuff.
I think that a few people sent contributions to a charity in their name.
Absolutely not, do not bring a gift.
Is it someone very special to you? Make a charity donation in their name and add that note to a card.
I hate it when people bring gifts to a no gifts gathering. It’s awkward for all the people who did as they were asked, who then might wonder if they should have brought one. It’s awkward for the honorees, who then have to thank the person as though they did something special.
If there is a special token (such as a framed photo, mentioned earlier) for Pete’s sake, give it to the couple quietly, preferably away from others, and keep it small.
I always bring a card. That’s not a gift.
Do they have a charity they love?
As I get older, I don’t want any more stuff. I would, however, be touched if someone made a donation in my honor to something I care about.
Thanks to everyone for your thoughts. If I gave a gift, it would be something I came across on Etsy that I thought they’d like, but this was before they specifically requested no gifts so I have decided to respect their decision and not give a gift.
You always can give them a gift any other day of the year if you come across something that you love. 
This situation is the perfect one for my go-to solution: write them a heartfelt note telling them what they mean to you! It is certainly a gift, but not a gift in the sense usually meant by the “no gifts” edict.
Another thought about bringing nice bottles of wine or spirits to No Gifts parties, especially when the party is at the honoree’s home…
For my husband’s 60th, we had a private chef come to our home and make and serve a special dinner for 8. That was his gift from me. I said no gifts, but they did all bring a good bottle of wine, which I was fine with as my husband loves wine. The wine they brought was mostly drunk that evening, as was a lot of other good wine.
I think bringing a bottle of wine to someone’s home is not necessarily a gift because it’s not polite to show up empty handed (hence why I always bring a card for a birthday or anniversary.) I guess there can be grey areas. In the situation the OP described, I wouldn’t bring anything but a card. OTOH, is it a 50th birthday pool party starting in the afternoon and going until late at night, but the invite says no gifts? I’d bring a bottle of something for sure, but I wouldn’t put it in a nice gift bag with ribbons and a label.
Agree, we always bring a bottle of wine whenever we are invited to dinner at someone’s home so it’s not something we see as a “gift”. If it was a celebratory party we’d probably bring a bottle of champagne. (But we wouldn’t bring alcohol to a wedding.)
To me a bottle of wine is a gift. How is it different than flowers or a box of expensive chocolates or whatever???
As a non-wine drinker household I also dislike when giving a bottle of wine is a fall back choice. And no I don’t want to save it for guests! 
Oh, we wouldn’t take wine when the hosts don’t drink it. But we always bring something for the hosts, which we see more as a “thank you” than as a gift. So yes, if you invited us we would probably arrive with a bunch of flowers
I don’t drink. My husband may have a beer or wine at a wedding. I hate when people bring us bottles of wine. Depending on the event, fine if they open it. But otherwise, we’re trying to pawn it off on someone afterwards. We recently cleaned out our kitchen cabinets in preparation for remodeling. 20 assorted bottles of wine and booze we never used.
But do you bring it if it’s a “no gift” event?
No issue with bringing a host item for an invite - but if no gifts is requested then come empty handed except for a smile and hug.
When I see “no gifts,” it means the host has so much stuff that they just don’t want another item in their house or a trinket. If I were to give a gift it would be something very sentimental, consumable or experience.
An “event” is something different usually, as I mentioned earlier we would likely make a donation to a charity that the inviter supports - it’s not a gift to them.
I have often brought a card to these events. And in there I say that I am making a donation to their favorite charity. For birthdays and anniveraries, I often get these card/gifts which people get a kick out of.https://www.amazon.com/dp/1939380359/ref=sspa_dk_detail_2?psc=1&pd_rd_i=1939380359&pd_rd_w=8SAp2&content-id=amzn1.sym.8c2f9165-8e93-42a1-8313-73d3809141a2&pf_rd_p=8c2f9165-8e93-42a1-8313-73d3809141a2&pf_rd_r=ENSBXWKJ22AEZMBKWK5T&pd_rd_wg=ZbSf2&pd_rd_r=89339c23-061e-463c-b8f2-7bf40b3b4c2b&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw
I love those cards!
It’s nice because it’s a “card” that doesn’t seem like it violates the “no gifts” rule but you feel like you are bringing something if other people are.
I guess we are delving into a “hostess gift” vs a gift for a party that states “no gifts.”
Most people wouldn’t specify “no gifts” if they are just inviting friends for dinner. Some times instead of wine I will bring a small box of chocolates or flowers. That’s a hostess gift though.
My brother once sent me one of those kind of cards - with the wrong year! When I mentioned it, he said “it’s the thought that counts.” Yes, the thought that you forgot when I was born, which was just under two years after he was born.