All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The son is not obliged to spend on the wives of his father –other than his mother –but if he spends on them, especially when are in need, then this is good as this is a kind of being kind and dutiful to his father by keeping ties with his wives.
However, the Fatwa we adopt her in Islamweb is that the son is obliged to spend on his mother and his sisters if they are poor and he is financially able to spend on them. For more benefit, please refer to Fatwa 85715.
Allaah Knows best.