what a hard thing to have to remember and deal with
the most important thing is that it wasn't your fault, you were only 8. you had no idea something like that would happen when you and your sister went out to the barn. it was molestation because you were 8 and no 8 year old should be made to do that. you weren't forced but you were coerced which is no different, please don't try to think you should've/could've stopped it, you were just so young
i'm so so sorry that happened with your parents, love and support are what they should've given you
if you remember and your sister remembers than maybe the best first step is to talk about it together. let her know you've been feeling guilty about having brought her with you, she may really want to talk about it. if she was younger than you and you were only 8 than she could also have some guilt going on - not having stopped what he was doing or helped you...
you wonder if it's your fault or his... you should talk to your sister because she might think it's hers.
i'm just throwing things out here though. another good idea would be to talk to a therapist about it.
personally i've had an easier time talking with my sister than a therapist
((hugs)) <3