Being family you may simply need to remind them of the fact that just because you are related it has nothing to do with your daycare relationship and they are bound by the same rules as the other families which means failure to abide by the rules of the signed contract means their continuation in your daycare is in jeopardy. If it involves the child's behaviour or something they are doing or not doing then turn it around on them with the idea that child is not having a good time at daycare because they have to sit out when we have movement time because they don't have running shoes as required by the contract or missed out on whatever because. Put the blame back onto the parent but then it becomes an issue between parent and child they can fix and is no longer about you caught in the middle.