And yes, be prepared for the fact that Jenny may choose to skip your party and spend time with her sister.
Well, Jenny has a previous commitment. And since she can't reconcile them, it would be rude for Jenny to leave her sister alone in order to go to *any* social engagement.
So it's not really "Jenny choosing to skip the OP's party"; it's "Jenny has plans for that night already, and it would be rude of her to change them."
Good point Toots, I didn't think of it that way.
Although this raises another interesting question - is it ALWAYS rude to leave a guest by themselves to attend another function? What if your guest is staying for a month? Are you really expected to decline all social invitations in that period, so that your guest is not left alone? Or what if your guest has other plans for that day, anyway? Is it ok then, to attend a social event?
My take on this is that if you know you are going to have a house guest on a specific date, and you get an invitation for that date, you respond with "Oh, thank you very much, but Great-Aunt Sarah will be staying with us that week." Leaving the host free to say, "Sorry, we'll miss you, " or "Oh, I've always wanted to meet her! Please, bring her along!"
If you have already accepted an invitation, and then you find that someone wants to come and visit you that day, what I'd do is tell the guest, "Sure, you can come and stay that weekend. But I already have plans for Saturday night, so you'll be on your own then." I don't think that's rude--you have explained the parameters of the visit and the guest is free to accept them or reject them. Or you could tell the prospective guest that the proposed dates for the visit simply don't work for you.
It's the prior commitment thing. In the first case, your first commitment is to the houseguest you have invited. In the second case, your first commitment is to the host who invited you.
Sure. But what if Aunt Sarah isn't staying for a week, but rather three months? Are you still expected to have no social life (outside of Aunt Sarah) for that period?
If it's three months, it's time to have some kind of party of your own so your houseguest can meet some of your friends, unless your guest is so reclusive/introverted/overwhelmed by other things that they don't want to get to know people, just spend most of the time with a book or computer. I can see that: a hypothetical friend or relative who needs a quiet place to finish writing a book or study for the bar exam, with occasional breaks for exercise or mindless entertainment. Not everyone would offer that person the three months in the spare room, but that's between the possible host and guest, and "I mostly want to do my work, and spend time with you over dinner a few nights a week, and I know how to do dishes" might work.
More likely, though, Aunt Sarah either already knows people in the area and can get together with them on her own, or is interested in meeting people, so the host might throw a party or board games night, or suggest a movie night or trip to the beach or such.