It's good of you to be supportive of your MIL, despite the fact you are ni longer married to her son. If she is someone you care for, then that is all that is important. It doesn't sound like her own kids are the most caring people. Avoid them as best ypu can, but be true to the concern you feel for your ex-MIL. They will try and distort your intentions, but don't worry about that. Avoid any unpleasant interaction with the son and daughter, as this would just be upsetting for their ill mum.