Her four sons and daughter were all living with the Sun family. She herself had to live with the Sun family. Why would she not have the Sun family's best interests at heart?
She had agreed to the split at first because she wanted to keep the things for her own little family.
From the first, she had hidden the things at her parents' house because there was someone to keep watch there, so she did not worry as much.
Although they knew that the second brother had some of his parents' things, they did not know how much he had. Besides, the second and third brothers had many children. If they were to split their inheritance, who knows how much would be apportioned to them. So with that in mind, they hid a portion for themselves.