Speaking as a parent, it is sometimes difficult to let go. Parents put a lot of love and hard work into bringing up an offspring and it doesn't end the day they leave home. They always worry about their children. Unfortunately this sometimes goes too far, as in your case (and mine). It is my experience that once you get parents that disapprove they rarely change.

As a grown-up offspring I think you can help your parents (and yourself) most by showing them that you are living without them successfully. It might mean that you need to make a stand by saying that you will only come to see them if the boyfriend is accepted and made welcome. It might mean you asking your sister to step back and leave it to you and your parents to sort out.

As regards this summer? Go take a break away from all this hassle enjoy your time with your boyfriend. Then deal with the problem of your parents when you feel ready.