What is the ruling on a wife shunning her husband's sisters without cause, knowing that she is observant of prayer and fasting?

This sudden behavior might be due to a psychological condition or another reason. It is incumbent upon the neighbors of this sister to treat her kindly and not ostracize her. Maintaining kinship ties and showing kindness to neighbors are required, and not as a form of reward or reciprocity, as the Prophet, peace and blessings be upon him, said: "The one who maintains ties of kinship is not the one who reciprocates, but the one who maintains ties of kinship is he who, when his relatives cut him off, he connects with them." So, the highest degrees of maintaining kinship ties and showing kindness are those that are without reciprocation.