Divorce Support Forums: Neglect

I fear (well know), that my Step-Daughter is being neglected by her Mom and Step-Father. She is 6 and sleeps in a playpen still in her Mother's Room, at the foot of the bed. My SD has a bed, that she would sleep in, but her two year old little half sister, is destructive, so-they make the 6 yr old sleep in a playpen, instead of the other way around. My SD is in dire need to go to the doctor for some health problems (i'll keep private but they are pretty severe), as well as get her eyes checked. Her mom provides the insurance, and we've offered to take her ALL THE TIME, just so it gets done for my SD's sake!
She tells us every time we have visitation that her step-dad just plays videos games when he gets home from work, and that she has to take care of the other kids, change their diapers and such (yes there's more kids). She is incredibly neglected in learning things-like letters and numbers, she can't count, she isn't enrolled in school, doesn't play with other kids, besides her siblings, which are 3+ yrs younger than her! She's told us she isn't allowed outside, unless its at Wal-Mart, and is only fed Spaghetti-O's at her mother's house, along with Dry Cereal. Also, her upkeep is almost unbearable. Her clothes smell, her nails are LONGGGG (finger and toe) and her hair is a complete mess!
I feel as if the Mom could care less about the child, and only wants child support money which is way more than we can afford. But we pay it any way to avoid problems obviously.
What can be done as far as getting custody of her, and are their any helpful tips to achieve that? We don't want any type of support from the Mother at all, if we get her in our full custody, but will allow visitations. We're just better than that.