wow, even grown adults can act like such children. Life isn't always fair, I can't believe a grown adult doesn't realize that. I think your son is going through a rough time, children are expensive and I think you're trying to help him out. I'm not sure what or if any help your grandson's mother gives. I think your daughter is in the wrong.
Leave them alone. Key word here is grown----even though it bothers you. Your daughter will have to sort things out on her own. Her brother will understand. They are going to have a moment that will change their lives forever.
It SOUNDS as though your daughter is pissed off at you for favoring her brother and his son. That is quite different from being jealous. That being true, deal with your daughter's probable feeling of not being worthwhile, lovable, and accepted.
Don't get involved it is their problem and they will decide when they want to work it out. Just establish a ground rule that when they are in your home they will act like civil adults. Maybe then they will take the hint and work out their differences.
treat them separately but equally things will straighten out between them
just call her every once in awhile out of the blue and tell her you love her, or give her a hug when you see her
tell her the times, were harder now. it,s getting beter.
Try to treat them equally but I know thatn't easy some times. You can always remind her that you will take her child on the next trip you go on.