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?
If that includes commentary, Seems that this one was:
The DPPN has a list of 47 Tissas. This seems to be #14 and the DPPN attributes v39 to him, saying “but see v1162”.
#12 (arahant from brahmin family of Rajagaha) has Thag vv.153-4
#13 (apparently king turned monk) has v.97
#23 (brother of Asoka) has vv.537-546
I don’t know to which extent this information is reliable, but it seems to be the traditional view.
Yes, thank you, wonderful! This answers exactly my question. (I should have put this in Q&A actually—going to move it there now.)