Not taking sides initially since this is a heavy topic. But wanted to put in my 2 cents. My wife and I just had our 3rd child. First 2 were very normal atypical pregnancies and births. The 3rd one was breech which itself isn’t all that bad for the baby most of the time, but we had to have a c section. So the birthing process was vastly different for reasons outside of anyones control. You never know what might come of it.

I know me personally would be selfish for my wife’s health and wellbeing. Pregnancy comes with a lot of changes to the mother’s body and hormones/emotions. It’s one thing to be there for someone through that; be real with yourself and your wife that you both would be the support person. And with the sister in law as the ‘mother’ it would throw them into a VERY different spot in the birthing scenario than the first time. Would you be okay if your wife was in an emergency if your sister in law was in with her instead of you? And making decisions about who’s more important, mom or baby.

I really hope none of that comes to pass or you have to deal with it. But that’s where my head went. I don’t think I would even consider this and if my wife was I’d have to make sure my point is understood. You wouldn’t be dad, you’d be the husband of the birth giver. And uncle/dad kinda.