They have stronger feelings and more of a sense of entitlement towards the inheritance than the sibling(s) that are doing the hard work of caregiving? It is beyond disgusting that they literally will not help at all, yet come back for the inheritance and represent themselves as caring adult children. How have you come to peace with this situation, if you have "crappy" siblings"?
Is there a will? Has your parent (your mom?) given any direction as to who should get what? Can you hold them off by saying, "now is not the time for that discussion"? And then talk with your parent about a plan or her wishes?
In all honesty, I would probably be considered one of those "crappy" siblings. My brother cares for my Dad and I have recently asked for a few mementos. In my case, however, there are two issues: 1) My brother, who is inheriting everything, is asking for my financial help to pay for the buriel, and 2) he has a long history of pawning things and has already had someone in to look at one of the antiques to determine its value. My asking for these things (5 total) is my way of preserving them to be able to pass them down to my children (he has none) before they are sold the next time he needs money.
In know this may not be the case in your home, but wanted you to know why my perspective might be a bit different. Again, sorry for you having to worry about this now. Hopefully, if you tell them the leave it alone for now, they will be respectful and will.
Sometimes, the reason the irresponsible sibling who refuses to help with the ailing parent, acts that way is due to the way they were raised by the parents. Often the child was coddled and raised to be irresponsible, lazy and greedy. I've seen it before. So, when they reach adulthood they have no reason to behave like a caring person. They were allowed to be a jerk from childhood. I blame the parents for doing that. The problem is that by this time, it's too late to fix it. You have ailing parents who only have the responsible adult child who will step in. The uncaring adult child is useless and could care less. What are supposed to do?
And often, the irresponsible adult child has already blown through more than their share of the estate, so there may not be much left when the parents pass away.
Life is not fair. We can only do what we think is the right thing and move on. Some people go to extremes to stay close to siblings, no matter how toxic they are. I'd weigh out the facts and do what I felt comfortable with. And I'd set up my Will to make sure there is no way they would inherit from me.
I think it's important to learn to let things go. There will always be people in the family who are troublesome. I'd learn when to fight them and when to walk away. At some point you have to decide if they are worth it.