Is it permissible to boycott a sister's children if they are causing problems and do not want their maternal uncle around? And are a sister's children considered kin (Rahim) such that the maternal uncle or aunt must maintain ties with them?

The severing of ties of kinship is forbidden, based on the hadith: "Do not envy one another, do not turn away from one another, and do not cut ties." And His (peace be upon him) saying: "A cutter will not enter Paradise," meaning one who severs ties of kinship. Relatives must be patient with the harshness of their kin and treat them kindly, in fulfillment of the hadith narrated by Abu Hurairah, which states: "If you are as you say, then you are feeding them hot ashes, and you will continue to have support from Allah against them as long as you persist in that."