haha..sorry to hear that bro...It's funny for us to hear but I understand how awkward it is..I think you should apologize coz they are gonna feel that you disrespected of their presence in house even though you actually forgot, so If I were you I would talk with her father and have your wife talk to her mother so it's gonna be more easy to talk... but I would strongly suggest you to talk, don't just avoid it..it's easy not to take responsiblity of what happen but that's always gonna haunt you as your big weakness...so good luck.