I go back and forth on which of these I think is the Nishigori side and which is the Toyomura side, but I have come to the conclusion that one pair of the triplets' grandparents own or owned Ice Castle (and if fewer than both of them own it now, it's because one or both of them is dead and left their share of rink ownership to one or both of Yuuko and Takeshi)
and one or both of the other pair of the triplets' grandparents worked at Ice Castle when Yuuko and Takeshi were kids (and if both, and/or if one or both of them skated there a lot, then their love story revolves around that rink as much as Yuuko and Takeshi's does, for the exact same reasons), and that's who taught Yuuri's first skating lessons
I think we found out Takeshi's and the triplets' ages in deuterocanonical something or other, not the show proper? I know there's an official art, I think from the fanbook, which lists the ages of Yuuko, Takeshi, and Yuuri as of the earliest flashback as 8, 7, and 5
Yuuko of course we heard is two years older than Yuuri as of March in ep 1, when Yuuri is 23, so we can pin Yuuko's 26th birthday to sometime between that March day and Yuuri's 24th birthday in November, probably on the earlier end because the same official source that gives us her maiden name says she's 26 and if her birthday is during the first few episodes then she's 26 for most of the series
Takeshi's 25th birthday must also precede the 18th anniversary of that earliest flashback, while being after Yuuri's 23rd birthday, but could still be on either side of Yuuko's birthday
I like the idea I've seen in several fics (notably @diedraechin 's but I've no idea who thought of it first) that the triplets were born during the 2010 Olympics, because it entertains me greatly to envision Yuuko, late in pregnancy and knowing triplets are usually early but still not expecting to go into labor until March at least, sitting in Yutopia dining area because they have the better TV, with Yuuri because Yuuko and Yuuri both wanted to watch the men's singles live
(and the Ice Ado trailer pins the 2006 Olympics to Paris, not Torino as in RL, which means Lambiel's YOI Torino Olympic silver was either 2002 or 2010; because in RL he came in 4th in 2010 Vancouver and 15th in 2002 Salt Lake City, it seems much likelier that Lambiel's YOI silver was from 2010 Torino)
and it's like three in the morning because time zones, so Takeshi is sleeping in Yuuri's bed (he'll watch the replay later) and Takeshi sleeps like a rock, and Mari and Hiroko and Toshiya are all asleep in their own beds and Yuuri won't dare risk they're not rested enough for work in the morning
and that is how Yuuri ends up missing watching Victor's Olympic performance live because he's holding Yuuko's hand while the triplets are being born
and because Yuuko is exhausted and Takeshi isn't there (and Yuuri is too stressed to process that the people asking him questions are assuming he's Yuuko's husband and/or the triplets' father), that is how the triplets get legally named what everyone was calling them during the pregnancy
...and given the history of the Victuuri romance, Victor on hearing this story decides it would be really romantic if he and Yuuri named their first kid Flip
(Yuuri, who has at some point witnessed if not personally experienced what happens when people think somebody's name sounds like something that's funnier because it's more hurtful, has better sense than that, but he does agree it would fit the pattern and be really romantic, so the kid ends up Filipp or Filippa)
(Victuuri kid 2 can be Sara or Sakura or idk, something that makes it vaguely plausible they'd go by Sally or Sal, which are also conveniently short for Salchow, or constructing the nickname by Russian methods gets Sasha which is already short for Aleksander or Aleksandra)
