I feel so horribly for you and my heart aches for your kids. It sounds like they are in great hands, but I know they are so confused and missing their mother.
Her excuse to her family of abuse shouldn't hold up well. I'm sure she's saying it to try to not look so bad to them. No good mom would EVER leave their kids with an abusive person... so this would mean, more than likely, that she's either lying or doesn't care about them... or both.
I wish you and your children the best.