Being Maternal

I consider myself to be very maternal- I had my son when I was 19- and was very happy to be a mom- I am a nurterer- of children, animals and adults at times- I never once contemplated not being a mother- and could never understand women that would have children and then later walk away from the responsibility- I dnt mean teeage mothers who are so young that they can’t handle the responsibilty alone, or women who choose to give their child up for adoption to a childless couple- both of these circumstances take great courage.
My stepdaughters mother- who had her at age 36- has taken it upon herself to move out of the county 40 miles away and to cut her time from 50% to maybe every other weekend if we are lucky. She calles every couple of days to talk to her daughter- only to talk about herself and never asks her about her school or her personal life at all- she has no involvement with her schooling and is not even offering to help pay some kind of extra support-
Instead of coming to get her daughter this Friday, she is going to a concert with a “friend” - one of many boyfriends that have been in and out for the last 2 years- I think it’s selfish and mean to tell your child that you are going out somewhere without them instead of spending time with them when you havent connected in 2 weeks.
I don’t understand it as a mother-
anyone have any advice?