Quoting mommycaressa:" well there pros and cons to it it might be the best thing for the baby but what about the older child ... [snip!] ... how is the older sibling gonna react when they find out that their mother gave away their sibling its just a really hard topic"
Well, hopefully, they are both raised to know they aren't alone in the world. They will be raised to know their mother did what she could for them. They might still resent her, they might not. She is doing what she has to do because she has realized she cannot care for both children.
They will understand in time. It's not as if the mother is placing the baby for adoption like s/he is nothing. She is going through a lot right now and instead of judging, you should be there for her.