In fairness, I wonder if this is part of the reason they have chosen to leave Saru on Kaminar.
As the first Kelpien to leave and to join Starfleet he should undoubtedly have some place in their history, even if his sister did choose to stay quiet about the true eventual outcome -- and even that is worth exploring in flashbacks or archive recordings, maybe even locked archive from Siranna that only he can open...? A lot was made of the emotional parting of Burnham and Spock after briefly being reunited, but the same was just as true of Saru and Siranna -- and they have an opportunity to review this here from the other side.
In many ways I hope they don't over-play the idea of Su'kal being a distant descent of Saru's family (whether Siranna or more someone else). As nice a touch as it might be, that just would just reinforce (yet again) the whole "small-universe" problem in Trek.
I guess Saru would have to be careful though, as revealing who he really is could cause all kinds of trouble within the context of the ban on time travel (!).
It definitely sounds like a decent story-arc that is well worth exploring and I'm sure they could squeeze in some events where he would need to call on the help of Capt. Burnham and the Disco-gang...!