Breastfeeding

We would like your advice on the following: My wife had a very difficult birth and after that she had to have a ceaser and a hysterectomy, so we wont be able to have any more kids. This was our first child and she is a girl. A family member suggested that if she breastfeeds her nephew who is a year old (Her brothers first child) then at least our daughter will have company when she is older and she will have an older brother to look up to and she can still keep that relationship after she reaches puberty since he will be her step brother. I have a few concerns regarding that like, what if they want to get married, and we also stay very far away from each other (720km) so it seems a bit senseless doing that. Also our upbringing styles will be different, we will probably have a more Islamic outlook and they will have a more modern outlook.