Is the second wife sinful for preventing her children from visiting their siblings? Is the husband sinful for complying with her? And is this considered severing ties of kinship?

It is not permissible for a mother to prevent her children from communicating with their paternal half-siblings, unless she fears for their religious commitment, because preventing them severs ties of kinship. There is no obedience due from the children in this prohibition, as it is disobedience to Allah. The father must convince the mother of what is right and bear the responsibility of raising his children to communicate and foster brotherhood. The paternal half-siblings should be keen to connect with their siblings, even if they have to communicate with them at their places of study. For the one who maintains ties of kinship is not the one who merely reciprocates, but rather the one who, when his ties of kinship are severed, reconnects them.