The arrangement as it stands is best for the child, he knows where he is going to be each weekend and is in a regular routine.
Ideally his Dad would have him, but if he is happy with one a fortnight visits, it is him who should be criticised.
Speaking as a new Step Nan, i am glad that my Grandson's Mother is of a similar opinion, i get to have my SGS every Saturday and then take him to his other Nans on Sunday afternoon. He is well loved by us all.
She allowed me to join in her pregnancy and wanted us around whilst she gave birth, as well as every visiting time afterwards. None of the "no visitors for days" senario, often advocated on here.
Sounds as the OP doesn't want this relationship to be as it is, so that she can have a bit of free babysitting when it suits her.
Did you have your MIL join in your pregnancy? How where things after birth? Do you have your own Mum around?
MILs can't win, so they often stop trying and concentrate on the relationships that they are welcome in. It is up to them how much a part they want to play in their DD's child life, my own Nan was a second Mum to me, whereas my cousin's Mums favoured their side of the family.