I knew my boyfriend for 11 years and lived with him for 7 before he told me about his SA.
Even before he told me, I knew a lot about his childhood, we grew up in the same town. So I could tell that things weren't always right with him but I had always chalked it up to the problems I already knew about.
Honestly, even with all the stress of dealing with the SA and flashbacks and the discovery of his acting out, I think things are easier for us now than they were before he disclosed. He doesn't have to be vigilant with his secrets and that makes everything easier. I don't have to feel guilty and responsible anymore about his acting out and issues with sex. I love him more for trusting me and I think he loves me more for being there for him and not judging him.
But he also chose me to tell; when you say "I am going to have to tell my sister" I wonder if she's someone you want to tell. My boyfriend hasn't told any of his family members and I think it would be a mistake if he did; they are so self-centered and ridiculous that they'd take it as an excuse to absolve themselves of anything they ever did wrong: "It's not OUR fault that he has these problems it's the abusers..." without wondering about their own role first in making him vulnerable and then in having him in an environment where no one noticed and he was afraid to tell.