Is it permissible to restrict visiting the deceased parents' relatives to holidays only, given that some of them are indifferent to the mother's death and say that mourning is three days, knowing that Allah has forbidden singing and dancing?

Filial piety does not cease after their death; it continues through prayer and seeking forgiveness for them, fulfilling their pledges, honoring their friends, and maintaining ties of kinship that you have only through them. It is advisable to maintain kinship ties and forgive their wrongs. Their going to the party does not mean they are unconcerned about your mother's death, as mourning is for three days. Singing and music are forbidden at all times. If their visits are only on holidays, this is not considered a severing of ties in common custom, as long as communication with them is maintained, then there is no harm in that.