Mumof,
I have, not one, but 3! -yes, 3- SILs just like that, so I know exactly what you mean.
One of them is totally obsessed with breastfeeding. I now breastfeed DC2 (3yo) and DC3 (21months old) but it just happened, but I never BFed DC1.
I felt shit about it and needed support in my decision. Only a week after I had DC1 she emailed to find out if I was breastfeeding and upon my reply she wrote to let me know that WHO, UNICEF, NHS and whathaveyou cannot all be wrong about the benefits ot BFing and I'm letting my daughter down, who would have to endure a childhood spent in hospitals !!!!! A "congratulations" would have been plenty enough!
Maybe your SIL's text was generic, but then it shows her obsession with "natural" childbirth.
I had a natural childbirth. When I found next I was pg I went to my gp and demanded a promise for CS or an abortion! I loved both my ELCSs and if I didn't know any better I'd start a campaign promoting them.
Oh, and one of the other SILs does the DR thing. And it's not even her, it's her husband, she barely made it out of high-school. Which would be fine with me was she not so smug about being married to someone with a PhD. She forgets I have one too and I know they don't reflect any apsect of RL
OP, so sorry I have turned this into my own rant, but I know how you feel, please please ignore this woman, or stop biting your lip and smug-her-back