The one cardinal rule is to avoid negative talk to the children or within ear-shot of the children about their biological parents. Regardless of how the adults are behaving, resist the temptation to turn the children against any adults involved in the equation with negative talk,  facial expressions or comments mumbled under the breath that convey hostility or dislike. Over time, they will form their own opinions, and will respect you more for attempting to protect the image of their biological parents and to nurture those relationships.