I think that you need to try to separate business from personal and try to deal with each other as you would like to be dealt with. This should always be the case but when buying a stranger's home, it's easy to ask for repairs, lower price, etc. It may be more difficult when negotiating with a relative and animosity can develop.
Make sure that you keep an open conversation, disclose concerns and have an agreement up front to deal amicably with each other. You don't want to get your home and be alienated from a family member!