Hi Sheri,
You sound like you have made up your mind about adopting him :o) And that you are ready and willing to "take it on". Bless you and your family for even considering that.
However, a couple of questions to think through.....
1)Does Grandma "enjoy" raising him, but just isn't able to right now? You don't want to ruffle feathers just because she broke her foot. Or perhaps, she might feel "relieved" that someone else is stepping in to help her Grandson?
2)How would your sister feel with you helping her son? Would she feel threatened, or insecure? If so, it could make things worse for your nephew....?????
Those are my biggest concerns for you. Other than that, I think your nephew is very lucky to have you want him.
Our experience: My husband is 1 of 7 children :o) His youngest brother was adopted into their family when he was 3 or 4. He was actually my husband's cousin. What I wanted you to know, that today he is in his 40's and we see him often. He is well rounded, and loves his family. It has never been brought up, or even talked about, since he was a boy. Everyone has always treated him like a family member, that was the only "rule" my M-I-L had. She said just love him, and make him feel comfortable in our house and treat him like a family member. In her mind, after having 6 kids, feeding & bathing another wasn't any big deal :O)
Anyway, S., good luck with this. This will take alot of patience and love on your part. I am certain that you are an "angel" in this young boy's life!
~N. :O)