I’ve heard other people going on about adopted children having issues, even if they are still babies, like this should be a reason to avoid adopting a child needing a home.
Children who are put up for adoption by the state are usually put there for a very good reason. What dose your SIL want, to see these poor children past for pillow to post not knowing where they are going to end up and never have a stable home environment, not to mention the risks of neglect and abuse, physically, mentally.... If there is a chance these children can get a stable and loving home through adoption they should be given it.
Not loving a non-bio child is a personal thing, some people are capable of it, some aren’t.
I’ve known a few people who have been adopted. A friend of mine, a lovely lady who had the most amazing childhood, her birth parents were just kids, mum white, dad not, mums parents forced the adoption. My friend made contact more out of curiosity about her race, background and ended up with a nice relationship with her birth father who didn’t know about her but would have tried to keep her, but the reality was he probably wouldn’t have been able to cope even if he had. She had a far better life than being with him or his family.
When I was young, there was also a little boy our neighbours adopted, they had been trying for years, as soon as the adoption went through they fell pregnant. I always felt so sorry for the adopted boy as he was treat very differently from their biological child, it was almost cruel, it would probably have been better for them to find another family who wanted him. I suppose that’s where your MIL could be proven right on the loving the biological child more.