HUF MEMBERSHIP

Mother owns a residential site acquired from her earnings on which the second son has built a house from his money and let out on rental basis. The second son, his wife and their child can form HUF and can mother transfer property rights to the son or his HUF with written documentation to tax authorities without registering? House is constructed from the funds of the Second son and parents assistance. His wife has not invested a single rupee on the property. In such a case, whether the Second sons wife be a member of the HUF and can relinquish her rights over the HUF property through a documentation? If so, whether such document will have to be registered in sub-registrar office or is it enough if documented and document held by the second son. Second son and his wife are cordial and happy but as an abundant caution can this be done. I will be grateful if you could provide your learned advice. Thanks & regards, V Krishnamurthy Father