good that you made that call and checked things. I'm proud on you man!
Related to Jim's age when he and your brother brought you into sex plays I would say that both they were kids at that time.
Don't know what some expert would say on all this but I somehow can't find child to be fully responsible for such acts. Kids are not at all aware of sexuality and once when it came to their lives because they are kids it become overwhelming for them and it colors their thoughts, plays and everything else.
I know because I went trough all that.
I've never felt abused in real terms, but as kid I was introduced to things that were inapropriate nor digestible for 6 year old kid.
I've been sad when I've read about your play with them two in that shower.
In my case such behavior has been brought by some adult to one kid (my bro told me how was it) and then it has spread among us boys.
He might be good man now (and your brother also), but I would like to know broader picture about their past, how everything started and how finished (that all doesn't look so innocent to me) to be able to conclude if there is need for some more action. Such plays doesn't start spontaneously. Don't know, those are just my thoughts.