It is rude to ask. There's really no way around it--it's just rude.
You put your host in the position of having to refuse a favor without really leaving an easy out. You've put your host on the spot.
What you are supposed to do is say, "I'm sorry, I don't think I'll be able to attend, because my sister is visiting from out of town, and I can't leave her home by herself."
Then your hostess can -volunteer- and say, "Oh, please, bring her along. Extend my invitation to her as well."
Or your hostess can say, "Oh, that's too bad, we'll miss you. I hope you have a lovely visit with your sister."
But it's rude, rude, rude to -ask-.