All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Moratorium interest that a creditor receives against debtor`s procrastination in payment is considered usury/interest which is unlawful for a Muslim to claim or benefit from. This based on the following narration: Jabir said that Allah's Messenger (PBUH) cursed the accepter of interest and its payer, and one who records it, and the two witnesses, and he said: They are all equal.{Related by Muslim}.
Procrastination in paying debts isn`t punished by Sharia through imposing usury/interest. Therefore, it is forbidden to claim the moratorium interest. However, if that interest was ruled by court, then it should be given back to its original owner because Allah, The Almighty, said in this regard, " But if ye turn back, ye shall have your capital sums: Deal not unjustly, and ye shall not be dealt with unjustly."{Al-Baqarah, 279}. Yet, if that owner was hard to find, then that sum should be spent for the common good of the Muslim Nation, such as schools, mosques, roads and the like, or given to the poor and needy.
In conclusion, if that interest was ruled by the court and your(Asker)sister is poor, then it is permissible to give her from that money if its owner was hard to find. This is because, in principle, one isn`t obliged to spend on his sister. However, it isn`t permissible to give your mother from it because you are obliged to spend on her, and that should be sufficient. And Allah knows best.