It depends on what type of person your mother in law is. I'm not saying this is 100% your mother in law but if it's me, I'll be thinking.......
1. Understanding - know that it's her son's fault so dont want you to suffer. volunteer to take the kids thinking that you'll have better chance at finding a new life partner without the kids.
2. Mean - thinking it's your fault her son is missing. Trying to 'keep' her grandchildren by her side cos they are the 'only things' left from her son. & trying to kick you away cos you are 'nothing but a sow' to her.
But remember this: THE KIDS ARE YOURS!!!
She has no right to take them away from you, especially since it's her son that is at fault. By the way, he has been 'missing' for some time, did you make a police report (just in case he really dont come back, you got proper records to show when he disappeared so can apply for 'divorce' next time)?
You should let her know that you wish to have your kids by your side, but just need her help during the day to look after them so that you can find work or rest. Make your stand clear to her.