Hoping to gain custody of my nephew (sister ped away)...?

My sister ped away 8 years ago leaving behind 3 boys who's father bailed on them (he is a complete loser). The boys currently live with their grandfather (not my dad). The oldest, who is now 14, was kicked out of peroquiel (sp?) school, is flunking all his cles, has gotten into fights and is on meds for ADHD and mild seizures (his grandfather claims it's epilepsy but he has never been diagnosed as such and I've never seen it). My husband and I have been thinking of asking for custody of him for years now and I finally got brave enough to ask his grandfather if he could stay with us the rest of the summer. There are the issues of resentment with the other two boys, but my biggest concern is how it might effect our family (my husband, myself and our 11 yr old son) by adding another family member. We know we can provide a loving, healthy, structured, family atmosphere, but I'm wondering if anyone has any experience adding another family member. What issues did you deal with?