I don't know the whole story but I think that you need to cut your SIL some slack here. Yeah, she should have been more organized and communicative with your family but if she is dealing with one of her children going through a major issue, her head is probably all sorts of cloudy. Her first priority is probably figuring things out with that situation and she's most likely very stressed which is why she's snapping at others. It's great that you took the nephews for a while and I'm sure that the mom appreciates it whether she vocalizes it or not right now.
I get what you're saying. It's more about the pattern if behavior up to this point. She and BIL split several years ago after a volatile 10 year marriage full of verbal abuse on both sides, alcoholism on his part, and cheating on both sides. The kids have seen way too much of all of it. DH and I through this whole situation with my niece have been offering to help in any way we can, including taking my niece into our home for a while to provide her with some stability. SIL says she has made that offer to my niece and my niece is not comfortable with that. We have told my niece in the past and since this recent situation that she is welcome to call, text or come to us any time. But the fact of the matter is, SIL uses my niece as a babysitter for her three little brothers. Which is part of what has triggered this anxiety and self harming in my niece. She feels so much responsibility for her brothers and doesn't want to burden her Mom. Those are her words.
I think what is so frustrating is that despite all of our offers and all of our efforts, SIL continues to refuse our help. But then will call and complain that the kids miss us or want to see us or need something. We can't always drop everything and go get them, we have our child and our family, but if she gives us a specific something that needs to happen we will gladly accommodate that. For example, this is how the conversation went earlier in the week:
"The boys would like to see you, let me know when would be a good time."
"Awesome! Sunday would probably be better for us, P can pick them up at this time."
"Well....I don't know...I don't know where we will be...I'm not sure what my schedule will be..."
"Ok, well, will Sunday work?"
"Yeah, Sunday is good. But I have to talk to so and so about watching the kids."
"Ok...how about you call me and let me know by Wednesday where they will be and what time P can pick them up?"
"Ok yeah I can do that."
And then I did not hear from her until last night. I know there is a lot going on. But she doesn't seem to be able to verbalize exactly what she needs us to do and everything we offer is met with a bunch of excuses about why that won't work.
The point is, the situation sucks. It just sucks.