There is mention of “Tissa, the Buddha’s cousin on his father’s side”, in SN 21.9 and in SN 22.84. We also find three Theragāthā ascribed to a Tissa, Thag 1.39, Thag 1.97, and Thag 2.17.
Is there any information anywhere if one of the Tissas in the Theragāthā is this cousin of the Buddha?