I'll be the devil's advocate here.

In my situation, a relative who is about 7 years older than me, who I know molested at least two if not 3 young children in my family, is being very suspicious.

You know this. there was an investigation which proved this, or you have simply heard through the family that this happened?

The family things happend 15-18 years ago, but he has consistantly been "close" to young, elementary aged kids at the Christian school I attended.

If in reference to the previous question, if there was an investigation and he was convicted of crimes against children how is he now in a position where he is constantly close to children? I find this confusing to say the least. If there was not, and he has been in such a position for the 15-18 years since and there has been no accustations of runors since, it's curious. Particularly since pedophiles, as mentioned elsewhere, have a very high rate of recidivism.

His baby sister didn't speak for the first 5 years of her life when he lived at home, I'm convinced this has something to do with him abusing her. She is now 15 and has never said anything about it.

Because there could be no other reason whatsoever for a child to start speaking late.

His neighbor, who also works with me, came into my office and asked me about him. She said someone told her to keep her kids away.

Whom was this someone and what exactly did they tell her? Are there more rumors/allegations about him aside from those within your family?

He recently met a young, single mother online and has change two of his cover photos to photos of her two young kids that he has never met! I find this to be creepy, considering what it know.

Or he might find the picture cute or a million other possibilities, but you are going with it being colored by what you "know".

Now do I mind my business or warn the mother? I have no evidence against him, other than wht happened 18 years ago. And of course, accusing an innocent person of child molesting has serious consequences. It could destoy his life/career if I'm wrong and tear apart my family.

Would you say something or leave it alone?

It's pretty clear that you believe this woman's children are in danger. I think you should take some action if you think that is true. Perhaps as Evan suggested. But you should accept that if you are wrong, there will quite possibly be unpleasant consequences for you and all around.

When I was growing up we had a priest as a family friend. One of my brothers once made a joke about him. He said, "Oh, if I screw up in mass he'll make me do pushups with my shirt off."

As kids, my father had us do pushups and situps every morning and every night and often as punishment for things. The Priest was a frequent guest at family gatehrings and as such had been around one time when one of my brothers had gotten in trouble and was doing some pushups and he commented on how strong he was. My brother had his shirt off at the time. My other brothers, keying off the fact that priests were celibate - and thus either aliens or *shudder* the gaysss - took this and ran with it that Father had a cruish on him. It became a joke among my brothers based on this single instance.

Based upon nothing more than overhearing this kind of joking, some members of my family became convinced that the priest was molesting altar boys. Despite my brothers' protestations to the contrary and the explanations proferred, word spread like wildfire and the priest was tried and convicted in the court of public opinion. He ended up having to leave the parish.

There was never a single allegation formally made against him and never a hint of controversy before or after. But in that town, he's known far and wide as a child molester.

So perhaps, I am a bit biased as well.

If I may ask, how old was he at the time and how old were the two, if not three, young children mentioned?

Quick question.

You said it happened before?, so definitive proof exists that this person had done it in the past & this is an established fact?.

If so, then I think you are OK to warn the other person.

It's not like it's a random accusation against an innocent person (if they have done it before) - I'd be very strongly against it if the person had no history at all (as you may be ruining an innocent persons life) - but if they have done it before, technically they are not innocent.

If they certainly 100% did it in the past, I don't believe you have much (morally) to worry about warning them - it's not like it's a baseless random accusation - but the 100% certain thing is a must ^.