I have a Texas Standard Possession Order and live more than 100mi away. Can my child stay with my family on my weekend if I'm not in town?

Your order likely includes a clause that says you can designate a competent adult to pick up your child for your possession time. That should allow your family the opportunity to spend time with your child. If you send a competent adult to pick up the child and the mother does not allow the child to go, that could be grounds for you to file an enforcement. In some counties, you have to make sure to have the person show up at the exact right time and day. However, the mother does not have the right to prevent a competent adult from picking up the child for you.