Mooch of a sister in law to be and how to avoid her?

I'm a really laid back person, even if I don't like someone I try my darndest to get along with them. I've been trying to get along with my sister in law as long as I've been dating my fiance (9 years this July), and we're never going to get along lol. The simple fact is, the chick is OVERLY opinionated to the point where she's rude to other people about it and it embarrasses me, leaves $1 tips, tries to get any free thing she can, tacky, impolite, tries to ask MY mom not hers, if she'll pay for stuff for her (she's 30 freaking years old and has a good job there's no excuse), and she also tries to mooch off me to the point where my friends have got mad at her. She is bossy beyond belief as well and has to have things her way or else she'll tell her mom on me (I'm not kidding) and then her mom tells my fiance and I have to hear about it. That and I have my wedding plans all set and she CONSTANTLY tries to change them so they're more her taste. For example I had my cake all picked out, she says "Well I think we should go with this flavor actually". Drives me up the freaking wall. She's honestly not a bad person and I don't wish any harm on her, but I just can't stand the above qualities about her and that's just how she is. She keeps asking to get together and I know she'll just want me to buy stuff for her or mooch, etc...I want to just avoid her how do I do this without being too obvious? I don't want to hurt her feelings but seriously I'm miserable every time her and I go out (just the two of us) and give up. Help?