That might not be correct. You're assuming that the in-game "one partner only" rule is actually what happened. It's not. It's confirmed by Trevor in Curse and Judgment, Simon, Alucard and Grant in Judgment, Yoko in Dawn and the multiple manuals that Alucard, Sypha and Grant were all together at the same time in the final battle.
THOUGH if you're saying that it depends on Trevor finding Sypha or not, it makes more sense.
One more thing: The ancestry tree I posted here shows that there are more surviving Belnades prior to Sypha -- from which Yoko descends. She might be an orphan, but it's possible she had sisters or brothers.
More depending if Trevor hitches with Sypha or not, they were all present, yes. but the manual evidence goes kinda against Sypha having siblings other than other Magic users at that time, Theory suggests that Simon being born is inevitable, but his parents are time-rift dependent.
As Trevor+Sypha shipping, Sypha and trevor lived a more peaceful life raising young X Belmont.
But, Sypha being the Witch who took part in defeating Dracula, she was able to start her own clan, a band of magicians sanctioned by the church known as the Belnades.
Such is how the old ways go, Sypha couldn't really do both.