How should one deal with non-Muslim sisters and a non-Muslim father, and is it permissible to seek help from Christian sisters?

The marriage of a Muslim woman to a disbeliever is unanimously prohibited and void. Whoever deems it permissible has committed an act of disbelief. As for the children, they follow their mother in Islam. If this marriage occurred out of ignorance of its prohibition, then the lineage is established. However, if they were aware of the prohibition, then according to the majority of scholars, the lineage is not established. It is not permissible to ally with the two sisters who apostatized from Islam, but it is permissible to treat them kindly with the intention of inclining their hearts towards Islam, provided there is no fear of temptation. As for the disbelieving father, it is obligatory to accompany him with kindness and to invite him to Islam. It is permissible to seek and accept assistance from a disbeliever if there is no fear of temptation.