Depending where you are, a 19 year old and 14 year old is statutory rape. You can't legally consent in that case. For the courts to not pursue him on those charges, he must have been able to provide reasonable doubt about him knowing her age.

I think you were jumping to conclusions, as was your sister. You used those assumptions to lay judgment. That's why neither of you are judges, or else we would have a lot of wrongfully convicted people behind bars.

You should apologize and make it clear that you were making assumptions that were unfair to everyone involved. You tell her that you care about her and just want her to be safe and happy. Then you tell her that you will stay out of it from this point forward unless the courts determine he was in fact a rapist.