I think I have a handle on the problem.
First of all, the young women (not girls) are emotionally immature. Secondly, they should be handling their problems with one anotjer without either you or husband.that is far too old to be punishing tjem like little kids. Why get involved at all ? They are women, not children.
Secondly, i can guarantee you that, no, they are not grateful when you hand them cars when other adults work and save for their own cars. I have found that buying stuff for our kids...too much stuff that we do out of wanting them not to go without...tends to produce UNgrateful, spoiled, selfish adults who expect even more and think you should keep it up. And demand you do. When does the party end?
I never understood buying our adult kids cars, houses, spending money, great clothes etc. They appreciate things THEY earn and respect us more if we teach them that nothing is free.
We never had the means to buy our kids thingslike cars so they started working at sixteen, took care of their own bills, learned a good work ethic and never ever blast us for not giving them cars, a free college education (they did grants and loans) and they are all independent and out of the house, not asking for money. And they are proud of themselves. Even our autistic adult son lives on his own on his own dime.
My advice is to stop the cars and the other freebies, step out of their petty arguments and treat them like women who can buy their own toys and settle their own disutes. Giving too much with no expectations often causes them to think we are fools and does/can lead to estrangement. You need their respect. You need to set boundaries and stop Christmas all the time.
Why did you let one daughter tell you what to do about the other daughter? Do they often bully you?
At any rate, i think the free presents and the over involvement in their lives needs to stop. Certainly they are bright enough to get part time jobs and move out. in my opinion you are too enmeshed with them...even afraid of them and you all need distance from one another. A lot of space to remember that yiou are different people
Love them enough to allow them to buy their own things and learn how to handle sister tiffs on their own. Or you may end up with two smart daughters who are spoled rotten, bossy to you and even mean. It is inappropriate to discipline adult daughters for having an argument. Perhaps you still see them as little. They arent.
Distance yourself or you could lose them both. And dont over spoil them because they will not appreciate it. You CAN change how you interact with dsughters. We can all change if we want. Its up to you to do it. You do them no good doing things this way. Go to therapy?
No offense meant and I could be wrong, but your daughters, by your description, sound like entitled spoiled ten year olds. Its time to help their character, quit your second job and stop giving them everything.i feel thats a problem of yours that is passed on to them, not for their good. Or yours. And you matter.
Sending you love and light
and hope!