Is keeping girls away from their barren stepmother—out of fear for them—considered a sin against her, and is preventing a husband’s wife from approaching her co-wife the solution to marital problems?

Childbearing is a blessing from Allah bestowed upon whomever He wills, so a Muslim should not overstep boundaries with their sibling on account of it. The co-wife must be treated as a Muslim, for she has rights upon the Muslim woman, among them good conjecture, unless there are clear indications to the contrary. There is no harm in preventing daughters from going to her if there is fear for their morals or that they might adopt deviant behaviors. The husband must be obeyed in what he commands, so if he prohibits visiting his second wife, it is not permissible to disobey him. The husband must be obeyed in what is good and right, and the co-wife must be treated with the manners of a Muslim woman.