See it's a delicate situation. I'm not going to use the "P" word because it's more complex than it is made out to be but that's basically it. Your FIL carries a lot of baggage about daughters and what society assumes about love marriage. And you and your BF are facing the brunt of it. The clever way to go about it is to first see how the sisters situation plays out, give him time and patience and come at it again after a few weeks or so. Don't push your relationship but don't give up on it either. Your BF needs to be there for his sis right now, support her. Handle your FIL delicately. They seem stubborn and villainous but it's just the baggage in their mind. My FIL still thanks me and my wife for not giving up on our relationship when he was against it and convinced him to turn around.

P.s. the rider to the above is - If you guys are actually in love, don't get married just because you were going to get married, this is a good time to evaluate if the relationship is truly worth it.