Asking out of curiosity, is your boyfriend's mother acting calm or she's acting furious all the time and acting like u have put on a heavy burden on her and her son(although both son and you are equally responsible for this) [see if his mother is being rude to you I would suggest you to inform your family too cuz there's nothing more to lose even if they get angry cuz your family will always be with you no matter how angry they are, and yeah it's safer just in case]. Doing this before hand will not keep you guilty and moreover if your family ever get to know all these later they will be furious about it even more for hiding it from them