SimmieTiff I don't recall there ever being an in-law relationship choice in CAS. Tbh, you don't really need them. The only place an in-law tag will show up is in the family tree or sim profile. I haven't dealt with making a family like you describe in CAS, but I can tell you that setting relationships in the game can get difficult at times from efforts to start a new save by moving sims and lots from another save. There are times that no matter how I go about it that the game will not give me the correct relationship. So, I have to get creative to get some of the way I want them.
So, one way to get around the game being a pain in your case, is to just let the hubby to be with no relationship and then after placing them in a house, marry them. That way you get the marriage certificate to put on the wall too. 😊
Hope this helps.