I'm sorry you and DH feel caught in the middle, Millie. What a dilemma! Criticize DIL1 and you risk a rift with her and DS1. But brush her comments off as "nothing" and you risk hurting DIL2 and a possible rift with her and DS2. Wow.
I would have opted for sympathizing with DIL2's feelings and validating her choices, while saying nothing to DIL1. However, it looks like you've already committed to "saying something" to DIL1 if these remarks are made in your presence. If that happens, I hope you say only something like, "Well, everybody's different" or "Different things work for different people" or ""To each their own" - and then change the subject as fast as possible. DIL2 might prefer if you criticized DIL1's comments, but I think the consequences of that might be too negative, so I wouldn't chance it. Hopefully, no such nasty comments will be made in front of you, "joking" or otherwise.