mil won't let me pay her back

This is a fantastic problem to have! More than likely, they are doing it just because they love you and FH & enjoy contributing to your union! And for that reason, I definitely would not ask them why, as it may come off as a hurtful or skeptical. My mom was this way with my brothers wedding as well. She Contributed much more than the bride’s family, and it was simply because she was excited about the wedding and really enjoys being involved in the process. Also, my mothers “love language” is giving gifts, and that may be the case for your in-laws as well. I would just graciously accept their generosity and thank them profusely for it

They might just really care and be really excited for you guys. Hopefully there’s no strings attached meaning eventually they will ask you to add on 20 of their work friends on the guest list or something.

View Quoted Comment They have added something like 30 people to the guest list. People they're paying to feed, but people that have my fiancé going "why are they inviting THEM?!"

View Quoted Comment Heh Yeah, then if that’s the case you really don’t owe them anything. At that point they’re not exactly paying for your wedding. They’re paying so that their friends can go. Either way it’s super kind of them to pay. With 30 extra people of their friends I would hope that they would be paying

View Quoted Comment I've given flowers & acknowledged she's been doing more than my own mom (but my mom lives a few states away, so that's part of it), we're taking them to dinner in the near future. What else can I do???

Just make sure there are no strings attached and you and fiance get to choose what you want. Some people are just generous.

View Quoted Comment I think that’s more than enough!

They probably just like doing it and are excited!

Gift horse, mouth!

Just say thank you, and realize it's probably one of their love languages.