if this was me I would first take into consideration the time frame in which this situation happened and whether legal charges /prosecution was done. you said when he was 12. so I take it this person is no longer a 12 year old. laws and times change. what is now molestation may have been legally defined as normal childhood show me yours and Ill show you mine, play house and other fantasy/ sexualized play.

my point if this was me I would not be jumping to conclusions and letting this mess with my head or affect my relationships with the significant others family. bottom line no one can say this person as an adult is going to molest a child now. how many of us did things as children that we would not dream of doing today...stealing, lying and yea crimes like shop lifting, getting into fist fights (now fist fights and cat fights are called the crime of assault) does that mean what went on in your childhoods define who and what we are today and what we are going to do today.

the fact that the parents are all moving on past what happened years ago says to me what ever happened has most likely already been dealt with to the satisfaction of all involved at the moment that it happened (ie the parents of the two children involved, and the two children involved.) years ago thats how this was dealt with ..with in the family with only those parties involved in the situation.

As for how do you deal with this...well only you can say how you want to deal with this. you have options...

you can continue on and accept that this was years ago and had nothing to do with you, it happened before your time with this family.

you can get therapy to help you deal with this in what ever ways is best for you.

you can call off the relationship with your girl and her family

you can call off the relationship with the girls family (in turn this may or may not cause isolate and cause your girl emotional problems due to being split between you and her family, for me I would never ask someone I love to make a choice between me and their families regardless of what their family has done before I even met them. )

my point only you can decide what you want to do in this situation. me I would consider it water under the bridge and move on because the situation doesnt involve me. I would leave it up to the one I loved to figure out what she wants to do about it for herself since its her family problems