Bhagavad Ramanujar might be Alavandar’s daughter’s son. As Alavandar was a sanyasi, he would not have known about his lineage. Moreover, in those days it was difficult to maintain a close relationship with a daughter’s off-springs, unlike these days where one can frequent daughter’s home and talk to them often. Swami Ramanujar was studying in a Gurukulam at that time; have heard from our elders, there are many such explanations.