You don’t say whether your brother-in-law has a financial power of attorney or one for health care. If it’s financial only, he should have no power with respect to your mother-in-law’s care or who visits her. I’d recommend contacting the social worker or ombudsperson at the hospital to discuss this. If it’s a power of attorney for health care, your brother-in-law probably does have the power to dictate who visits your mother-in-law. But the hospital social worker or ombudsperson may still be able to intervene to mediate a resolution that works for everyone.