You have the advantage of knowing how they reacted to your sister coming out. If how they reacted isn't something you want then you don't have to tell them. It's your romantic life and they don't have to approve for you to be happy.

If you tell them know you have the peace of mind that it is out there and don't have to deal with it later. This is of course at the risk of it damaging your relationship with your parents based on their beliefs. I'm sure it would hurt a lot to not be accepted for what you are to your parents.

Have you talked to your sister about it? Maybe she can give you some advice since she has dealt with some of these battles.