YES! Most certainly talk about it. Maybe to your brother, if not the SIL. But, make sure you have some solutions that she might think about for taking care of her one year old. She may just have to pay for a sitter. It's better than paying for the damage that could happen to your mom. Or ask a friend from church, swap times with another friend, or if you can once in a great while. Has she considered taking classes in the evenings? Would her DH be able to take care of the baby then? It doesn't sound like this is a short term problem for your mom, and even if she were back to health, she may have to limit her movements in the future. I'm sure your brother and SIL don't want to be responsible for any further damage to your mom's condition. But, definately discuss it with them. Non-communication is miscommunication in many instances. Good luck, and I hope your mom starts to have some better days ahead.
J.