Your mother and sister being rude cult followers is painful and very annoying, but seems not really relevant for the question at hand.
The problem being, the behavior of the mother, her husband and Pale's sister, sets an example for Pale's child. Allowing that behaviour to continue, then indicates that behaviour is correct, justified and acceptable, and so how long will it take before the child starts to treat Pale in a similar manner?
Rules of behaviour need to be established now, not after a problem develops. The child's welfare rests with both parents, not the grandparents or Aunt. Both parents need to agree to work together in the interest of the child regardless of their relationship with each other.
The blood issue is a valid concern as JW's typically consider their actions as being what God requires. If the child had an accident while in the care of the Grandmother or Sister, and if blood was required, would the GrandM or sister inform the parents of required treatment or would they act to prohibit blood as a method for the child to have God's favour?
While Pale understandably wants his child to have a relationship with his family, that should not occur at the expense of his relationship with his child or at the risk of the child's welfare.