You are right in trying to alleviate your niece’s karmic retributions.
Apparently, your brother and his wife owe her. They also need help.
Your actions have helped to some extent, but perhaps they are not timely to have much impact. And I hope I am not offending you, but your gongfu is not that strong yet. It is like trying to chip away at a large rock with a small hammer. However, you could request help from people who are stronger than you and are trained in this kind of spiritual work.
The Chinese tradition has Dharmas such as Medicine Master Buddha (look for the explanation of the Medicine Master Sutra on our web site) and Past Creditors (rebirth Dharma).
When you have the chance to come to the temple, we can give you more detail and referrals.