Raising a baby not mine

Ok so we met this girl a month before she had her baby, only seen her the 1 time we met her and then when she had the baby we were at the hospital and have been there since day 1. The 1st couple weeks she lived with her grandmother and it got pretty bad there with an uncle who was a spazzed out drug head so we took her and her baby in. He is now 2 months old and we have literally been taking care of him the whole time (don't get me wrong me and my hubby love it considering ours are 12 and 15) but I worry day by day if she's gonna get upset or mad about something and just take off with him. She doesn't even stay here anymore. She stays with her boyfriend around the corner and is with him what time she isn't at work. She will come in a look at baby and be like oh mommy loves you and then goes about her business. She gets him all he needs so if we spend money on him its because we want to so there no problem there but when she does try to hold him he doesn't want her holding him. He has bonded with me and my husband. It just breaks my heart that she would rather be with her boyfriend and be drinking and whatever else than to be with this precious baby. The dad sees him once in a while and he took him one day and had him for several hours and he hasn't been changed the whole time and he had only had 1 bottle (which was the one he had when he left) then that night he got sick and the next day he had him for 6 hours and was changed 2 times and only had 6 ounces of pedialyte the whole time. They are young parents but my question is what should I do? I'd like to get something done legally that if something happens we have the chance to get him but I'm afraid if I bring it up to her she will get upset and leave. I just really don't know what to do. She only spends a total of 5 minutes a day with him and we took him to the cardiologist yesterday and he recommended blood work to check for a hereditary disease and she refused knowing her grandmother, dad, and her have it and it is life threatening. please help!!