Im probably too late at giving some advice.. but I just felt the need to try and help u! My dh hasnt liked a few of my 2 sisters boyfriends.. and actually had a punch up with my younger sisters ex boyfriend 3 yrs ago. I havent liked the guys also but i hate confrontation and i wld much rather pretend everything is fine n dandy just to keep things all happy.
We have had countless arguments over it bcause i just want everyone to play happy families but I am slowly realising tho that it is my dhs opinion and he shld be allowed to voice it. I am also slowly starting to realise that he is the bigger man for actually being able to acknowledge that the boyfriends arent treating my sisters right.. and i do love the fact that he cares about the way they are treated.. and really im the soft one for just ignoring what idiots the guys are!

I dont think u shld pre-warn ur dh.. i think it will give him more time to stew on it and get more angry! And it will kinda be made into more of a big deal if he feels like u had to prewarn him about it.. it will kinda put it in his head that he shld be angry IYKWIM??!

I think let the situation pan out however it is going too.. keep on pretending that u dont know. Have a chat to ur dh afterwards and just explain that even if he doesnt support their relationship.. its ur sister that needs to know that u guys are going to support her no matter what. Im sure she is having enough conflicting thoughts in her head about ppl not thinking its the right thing for her to do. Make sure u BOTH let her know that u love her and that she is going to be a great mummy

Does all this make sense??!!

Good luck this weekend