stepgrandchild vs,actual grandchild

a not so long time ago young, dumb, and silly me thought she loved a man and had a kid with him.only to leave him for being abusive fast forward 4 years almost 5. finally found the love of my life and made the right choices, he decided that my daughter was basically his daughter. he loves he and his family does to, but now that new baby is here his mother is treating my daughter a lot,different. to the point my daughter actually notices, we've been through this before first her real father wanted joint custody then just to watch her while I was at work, then suddenly hed not even show up to get her from me and she would end up hanging out in the employee lounge until someone could come,get her or the he would bring her back with 3 hours in my shift. so she is no stranger to being ignored or forgotten. which breaks my heart but she considers her brothers father her dad and his family her family too. but recently his mother has started ignoring her or not wanting anything to do with her. perfect example my daughter's name is Brinnlynn she does not go by anything but Brinnlynn and will tell you if your wrong. his mom just called her Brinn, then B. then that other kid. upon asking her what was up she said that's not her grand daughter so why should she pay attention to her? it's gotten t l the point my daughter is asking question. how do I explain to a 5 year old basically what's going on?