Is not speaking to ill-mannered nieces and nephews, while continuing to communicate with their mothers (your sisters), considered a severance of kinship ties?

The upright Shari'ah encourages maintaining ties of kinship. The mistreatment the sister receives from her nieces and nephews should be met with kindness and patience, in accordance with His, the Almighty's, saying: ﴿Repel [evil] with that which is better, and thereupon the one between whom and you was enmity [will become] as though he was a devoted friend﴾ [Fussilat: 34].

The Prophet, peace and blessings be upon him, informed the one who complained about the ill-treatment of his relatives: «If what you say is true, then you are feeding them hot ashes, and there will always be a supporter from Allah against them as long as you remain in that state.»

The scholars have permitted shunning those who commit sins as a deterrent, if the shunning is beneficial.

Therefore, the sister must return to communicating with her nieces and nephews and seek reward from Allah, while guarding against the whispers of Satan, for the Prophet, peace and blessings be upon him, said: «It is not lawful for a Muslim to abandon his brother for more than three nights.»