scg, my comment had nothing to do with being judgemental towards you. My thoughts were more along these lines: This is the second time you have had to step in to care for these children. You've done your best, and done very well, especially under the circumstances. The major thing I'm seeing is, your mother, and these kids have been through a major emotional upheaval. Losing a spouse takes an emotional toll on the surviving spouse, like it or not. In the meantime, 2 mentally/emotionally challenged children are going through just as much, if not more emotional upheaval as well. Routine, and normalcy as much as can be expected, even with your dad gone, are critical, but the girl needs more than just that. Instead of keeping things as normal as possible, with them going to visit the girl in the hospital, things are turned even more upside down in the boy's world.
That's not criticism, just observation. Again, you are doing your best, and stepping up to the plate admirably. The thing I see, is that you are the back-up plan, and that needs to be addressed. It's not really fair to you, your husband, or the kids.
Maybe it's time to talk seriously with your mom about this. Your mom is no spring chicken. There will be times when she will get sick, or events happen that she will be unable to provide the full care that both of them need, like is happening now. Instead of dumping them in an institution, or on you, it might be time to start investigating nearby group homes, or families in a similar situation. Maybe she would be willing to work with another family, or group home at times when they need someone to watch their kid(s), in exchange for the same thing when she needs it. That way the kids would already know who they're going to be with. In a group home situation, she could see if they allow short term care, as well as long term care, (just in case), for the kids, so the transition would not be so hard on the kids. Another advantage is that there is someone with more experience with the type of children involved.
Now would be a good time to start investigating options, since some of these group homes can be excellent, but others are simply a pay check, and the kids are mistreated. This gives her time to weed out the less than desirable ones, so the kids are treated good.
I know it's none of my business, but I'd hate to see them just dumped into an institution if things got rough, or your mom was unable to care for them for awhile. Longer term planning for their care should also be on the table as a discussion topic too.