D..
I'm sorry, but if you want to have a relationship with your son and grandchild, you have to endure his girlfriend's unbelievable behavior, whatever it is. Part of enduring is ignoring too.
Your son is right, that in order to have a relationship with him, you have to accept his family. If it were YOU and your husband, and your mom couldn't stand your husband, you would feel the same way.
Don't push so hard with them. If you aren't making your son feel defensive about this woman, at some point his eyes may be opened and he might decide not to have her in his life anymore. If they get married, maybe she will grow up and stop acting like this.
No more financial assistance, though. Other than babysitting, they need to learn to stand on their own four feet.
If you don't start letting her antics roll off your back, you will be miserable for the rest of your days.
Good luck,
Dawn