Can a mental institution refuse the parents to see their child?

My sister in law has temporary guardianship, she didn't tell us she put our 5 year old daughter in a mental institution. We found out by family and friends. We called the mental hospital to see if we can see her even with supervise visits. They said we can not confirm or deny them being the without a code which my sister in law doesn't want to give to us at all. I am writing to the director of the mental health hospital telling them they are violating our rights as their parents and that we have to prove that she only has temporary guardianship we still have our rights intact and that we would like to have the code to see our daughter and to talk to her doctor to see what she is on and how she is doing in school there, cause of course my sister in law told us we can know that when we get them back.