Probably not because seichi's sis has a kid and seemed probably married not engaged and senpai never said anything about a kid, the only thing was that his fiance was an owner of the hospital he worked with, the kurusu did own the place seto worked at but never confirmed if they had another location
*daughter of the owner of the hospital* it's probably a coincidence and pretty sure if nao were to meet the dad seichi would also have him meet his other family members before the wedding at the last chapter and he'll know if his senpai was among them but there wasn't any implication of that in the story
Probably a coincidence tho
Anyone catch that senpai is the finance of sechis sister? I'm pretty sure he is