All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The gift of a mother who is in sound mind of her home to her widowed daughter in-law is permissible and it is not in violation to the Quran or the Sunnah. As long as the house is her property, she has the right to dispose of it in her life and while she is in good health, and she is not obliged to seek the permission for her children or others, or to inform them of that.
Accordingly, what your mother did is correct in the scale of the Sharee’ah, and it is one of the righteous deeds for which a great reward is sought.
Allah knows best.