Agnes Tachyon seems to show up amongst the mates of the sluttiest mares quite a bit. Did he just have a thing for easy women?

It's a messy mixed bag.

You've got the 'vague feeling' type of familial shout-out. Whenever Bakushin and Kitasan Black interact, they'll both note feeling something 'familiar' about each other- Which never gets expanded upon, only really relevant to people who know Bakushin is Kitasan's grand-dad IRL. You'll also get plenty of interactions of Agnes doting on Scarlet, but they'll never just out-right say it's because they're kin.

Then you get the 'direct' types. Like the Mejiro's, who all openly acknowledge each other as family. Because other-wise it'd be hard to get around the fact that they're all wearing the same name. Goldship being related to McQueen is never brought up though (but is the writing room motivation behind having them mess with each other so much).

Then you've got the cases where the anime/game will invent family for the horse girls. But even those figures will tend to be references to trainers, jockeys or owners in some way, shape or form.