Dear Aunt Babz,
I must not have received part two.. and I am trying to understand the jist here. So bear with me.
To my understanding the SIL hasn’t stayed with you yet in the condo you rent, but has stayed with you before and the result was you picking up the tab and having to make a decision on leaving the place you were in. If I am wrong here, I apologize, but the letter here is unclear.
If this is the case, and you are feeling like you have to pick up the tab… what the heck are you doing? Sure, in your place it may be good practice to be hospitable and make dinner or what have you. But picking up the tab while you go out is unacceptable. When you dine out, simply ask the wait staff to separate the cheque before you order, so that she is aware she will have to pay for her portion and you are not to be used like that. It is a non-confrontational way to solve an uncomfortable situation. Maybe the SIL is under the impression that her brother (your husband) is willing to pay the tab. Maybe he has told her that he would. Be sure, talk to him and ask if that is the case. If it is.. you two definitely need to have a talk.
As for the wiggling out of leaving your condo when they do come to visit. This one is quite easy. If the question arises that the SIL wants to go elsewhere, simply tell her that you have rented the condo for the month, paid for it, and have made plans for other visitors so you cannot go this time, maybe another time when you can plan a little better.
Maybe if you and your husband talk and set down some ground rules it would be easier to make these comments as well. I know you said that he is easy going. But being easy going doesn’t imply that you have to pay for everyone, or that you can’t talk about it.
Make sure you are being rational and not accusatory about things, because this is his sister. Ask him questions like “hey, just wondering, did you tell your sister that we would pay for dining out while they were here? I just want to be sure of how much to set aside for this trip.” or “did you get a chance to ask your sister how much cupboard space she will need for her groceries the week that she is here? You really should so she can plan her budget for her trip” . These sorts of questions are non-threatening and will make for an easier conversation if he has made some sort of agreement with the SIL.
Bottom line is you are feeling resentful, and that isn‘t healthy. You need to talk with your husband and figure out what you need to do.