as far as the problem of the party - you could have one party at your dads with that side of your family and then have another party at your place with your mom's side.
That way you are in the clear with everybody.
As far as your sister - 1. She is not going to listen to you. That is the worst part. But has it occured to you to talk to the boyfriend - something just short of a threat about is illegal status and his age. I wonder if it has occured to him that he can be prosecuted for rape if your sister gets preganent with or without her cooperation.
There is always to possiblity the love each other but he is 19 and there are legal ramafications that he may need to be reminded of.
