I think it's natural for you to be concerned about whether your brother and SIL are making informed decisions rather than just doing what the Ob tells them to do without questioning it. And I think it's normal to be horrified if you think they are making uninformed decisions. I think it's a whole other thing to question and judge them for those decisions, to their faces at least.
I'm constantly saddened when I hear stories like this one, where I consider uninformed decisions are being made and women (and their partners) are subjecting themselves and their babies to interventions without knowing the full facts. My own SIL had a c-section because her baby was "too big" to be birthed vaginally - the baby was only 3.1kg in the end (smaller than my baby!), but her Ob "wouldn't let her go past 38 weeks" because he considered her baby was too big. Now, I was sad for my SIL and her baby that she made that decision, which I considered was an uninformed one, but I didn't tell her that. I figured it wasn't my place to educate her, and who am I to judge? (Obviously I am judging her, but not to her face! ).
I suppose what I'm saying is that I understand where you're coming from, and I think I would feel the same way if I were in the same situation. But I probably would bite my tongue - because I know what it's like to have people express their opinions about my decisions, even when it's none of their business.
Wishing little Isaac a safe arrival.