Is the wife considered sinful if she reduces her visits and fulfilling her duty towards her husband's only sister, while striving to preserve her dignity, especially since her husband prevents her from visiting her?

It is advisable to treat the wife's sister kindly, to be patient with her character, to show her benevolence, and to offer her light gifts, remembering the words of Allah Almighty: (Repel [evil] with that which is better). There is no sin in reducing visits or reciprocating treatment as long as it does not lead to abandonment. However, it is preferable to show benevolence and not to reciprocate treatment to avoid family discord and estrangement.