I'm not telling my girlfriend I used to hook up with her sister in the past
Why wouldn't you tell her you hooked up with her sister?
I'm scared she will talk to me less and I don't want things to change between us.
I understand why you're scared, but you should tell her. It's better to be honest now than to hurt your relationship by hiding it and having her find out later. Her sister may remember you and tell her even if you don't.
I think I did the right thing by not telling. She loves me and we got married, I hate being questioned about things.
Don't you think she deserves to know such a thing though? If she finds out later in life it will only make it worse.