This is so deeply sad.For everyone
Like dancer I have experience of a similar situation.
I as an adult accept things that confused me when a a child.My love was centred on my Grandparents.My feeling for my mother were as if she were an older sister.Her bother and sisters varied in their attitute to me.Two were wonderful.One hid her true feelings.Another OK.It was really the collective expressions that confused me.Only as an adult (and with pain) did I clearly see the whole picture.
I can say truthfully that my Mother loved me.In her way.She nurtured me in her body and gave birth.Indeed just after I gave birth I thought of that.But my reasoning was how could she then go through this then hand me over.When my Father left her I was 10 months old. within weeks she went out with her friends and resumed single life.There was no stopping her. So I conclude she could compartmentise her self.
I belonged to her past.
it may be that your DIL does this also.But her fantasy of being in the team and part of an extended family(the squad) has overtaken her duty to her baby.This did not happen to me.Nor most people but I know others have been like my mother.
also imagine if she had stayed and taken her frustration out on the baby.
Mothering is not the same for everyone.We have to respect that.

many of us have said our bit about this.I add these thoughts due to dancer sharing.
My opinion is that baby stays with you for a time until she goes to school..then your son has a new lifestyle..dont look too far..your son may have to adjust change his job.Mother may want visits..but all that is for later.
Meanwhile enjoy that child.Give her days filled with love music and approval.LET HER SING.