You don't have to have an excuse, you can be "busy" and it's very impolite to ask what someone else's busi-ness is? A cold, "I'm sorry, I don't see that that's any of your business" and continual cold shoulder might get the message to her that you don't want to see her without your saying flat out that you don't like her.

Your husband isn't on your side about whether you see/like your sister or not?

I'm sure you have your reasons for why you don't want to talk to or see your sister and they're yours so if others want to think you a meany or something, let them? You don't have to apologize to anyone, least of all to someone who is obnoxiously trying to contact you when it's obvious you don't want contact with her.