This obviously is bothering you. In order to enjoy yourself you need to clarify things before the event. Be honest and explain your confusion to your houseguest- someone obviously enough of a friend you can approach the issue with. Tell her you will enjoy getting together with the third couple but you are confused about things. Tell her you don't want to embarrass anyone with any awkwardness at the dinner and want to clarify if you should cover the birthday person's dinner as customary, or if the dinner is to be their treat entirely. Taking care of this now- even with a phone call just to discuss the issue- is the best way for you to get it out of your mind and relax. YOU count and need to be able to enjoy without resentment or wondering- your example of past behavior means you deserve clarification.