Seeking advice on strategies for brother, brother’s girlfriend/partner and Mom get along better.

My brother divorced 5 years ago and has had a new girlfriend/partner since. An agreement was made that my brother was to buy a new house for himself, girlfriend and our mom to live together. He chose a house that was not ideal for Mom. Not an inlaw (separate kitchen etc.). Mom voiced opinion that she didn’t think it would work. Brother continued with the house purchase. 2 days before house settlement, we had sibling and Mom meeting. I asked the brother not to invite girlfriend. Girlfriend came, Mom expressed opinion how living arrangements would not work, Girlfriend blew up- yelled, said Mom was going to nursing home and Brother threw phone on floor, girlfriend stormed out said she was “done”.

1 year later, girlfriend ignores Mom at family gatherings Girlfriend has not ever since the incident come over to Mom house – who is living alone. Brother comes over less to Moms house. And just to check up, little conversation. At family gatherings he stays with girlfriend, ignores Mom.

Mom is upset, cries often. I’ve talked to brother asking him to come by mom more and talk with her. It appears to me, that girlfriend has strong influence and influencing him on not come and help mom. I believe she still holds a grudge on not getting the “big” house.
Mom needs help. Mom is in 80s and should not be suffering. I agree that Mom has been rude and polite at times.
Question- I’m seeking recommendations on what I can do to help these people get along. Seeking ways to help mom and brother and girlfriend to move past the incident.