How to get legal custody of a grandchild that lives with me full time .

We have our now 8 month old grandson living with us since June. The father and mother are married the father work in a job that requires a lot of travel and the mother doesn't want to be away from her husband and chose to leave her children (we also have two of her children from her first marriage living with us) and be with her husband(our son) We don't think she is mentally stable and would rather sleep in a hotel than be with her children, when they do visit she has very little to do with the kids ( feeding bathing or spending time with them. It's just a photo opportunity for her to make her look like a good mom. The baby was 4 months old and is now 8 months old and bonded to us. When they did have the kids it was my son who was up fixing breakfast for them and he was up with the baby at night and it was my son whom her girls asked to tuck them into bed eventhouhg their mom was in the room also. My son pays his sister to baby sit the girls and I pay for the babies food and diapers.