They could undermine your parenting style, especially if they disagree with them. I know that was an issue with my mom when my grandma moved in. She didn't approve of the way we were being raised and tried to help out, which didn't go over well. They could also negatively influence your kids, I guess. But there are more benefits, in my opinion. I learned a lot from my grandmother, who had lived an amazing life. I think it's great when children know their extended family. They act as an extra support system.
Having other members of your family or relatives around the house can influence both negative and some positive behaviours. Also, the child could get more attention, I think this can make them find leaving the family more difficult in later life because they would be so use to the company. However, as Annie said, have more people around acts like an extra support system and can prove positive towards the child,