What is the religious ruling on the aunt and sister's handling of the orphan's (the questioner and his brother's) money and their seizure of a large portion of the inheritance without their knowledge?

Unlawfully devouring people's wealth is one of the gravest prohibitions. It is forbidden to consume people's wealth, whether they are orphans or not, though consuming the wealth of orphans is even more strictly prohibited. You can demand your right, but forgiveness and pardon are better.