Here’s another thought— what if the Rise Joneses are distantly related to the Hamato clan?
Future Casey is pretty darn identical to Hamato Yoshi/Lou Jitsu— same nose, same chin, same ears, same eyebrows, similar hair, etc, and it might just be the art style but his and Cassandra’s eye shapes look close to Karai’s. You could also maybe argue Cassandra has a similar hairline to Lou Jitsu
Rise Splinter broke off from his family because of how much their traditions hurt him. Odds are he isn’t the only Hamato to do so— maybe a cousin moved away to New York, married someone named Jones, and had Cassandra (or, alternatively, Cassandra was taken from her family as a baby/toddler, raised in an American clan, and was given the name Cassandra Jones to sever her ties to her birth family)
EDIT Or (and this is my new favorite theory, inspired by @webrokethe4thwall in the replies below) Splinter got around during his Lou Jitsu days and possibly fathered a few kids during that time. Since Cassandra is confirmed to be April’s age (so like 16 during seasons 1 and 2), and Splinter has been a dad to the turtles for like thirteen years, the timeline does fit for her to be biologically his (it would also be an interesting way to harken back to the 2012 series if Rise Splinter had a semi-antagonistic biological daughter alongside his adoptive/semi-cloned/However You Explain The Mutant Relation sons). Cassandra could’ve taken after her Jones parent and Lou Jitsu’s looks could’ve skipped a generation with Casey
Or maybe (since Cassandra admittedly doesn’t look that much like the Hamatos we’ve seen) Cassandra either had a kid with a Hamato cousin or used Hamato DNA to have Casey
Or Casey was adopted and his birth family was the Hamatos. (From what I’ve gleaned from listening to adopted people, it’s usually in the kid’s best interest to let them have a connection to their biological family, so they can have answers to any questions they might have). Either Cassandra called dibs on their orphaned cousin, or (if you’re a capril fan), an adoption agency figured since April herself is an honorary Hamato, they’d let her and Cassandra have first dibs on him (and Cassandra idk won the right name him after herself by winning rock-paper-scissors or something)
