-Please, please tell me that Clarissa isn't the one she looks like. They can't be serious with that cliche, right? RIGHT?
To answer this...well...its a spoiler, but probably not in the way you're thinking. If you don't mind being spoiled then, well...
SPOILERS
Clarissa is and she isn't Alexia. By which I mean she is Alexia BIOLOGICALLY, but she's not the same Alexia that people like Labyrinthia are acquainted with. That Alexia is actually is Clarissa's mother's ACTUAL daughter, while Clarissa is the ACTUAL Princess Alexia, however, due to an incident and their striking similarity in appearances (which apparently is purely coincidental; such things ARE possible in real life, for 2 totally unrelated people to have striking similarities to one another), they got swapped at a young age, and this factor was never discovered. The scenario is actually well illustrated in game, cause it gets around such questions like "wait, wouldn't the mother recognize her own daughter's outfit?" or "Why would those two HAPPEN to be in the same place?" END SPOILERS
The game never outright states this, but all the evidence is there, to the point where if its NOT true, then that's a shit load of red herrings they tossed out there...and hell, it not being true only contradicts the plot points and setting they built up related to a few things, so it'd be especially stupid to deny it.
Sort of like how Claude and Syliva if FE4 are strongly suggested to be long lost siblings, but then Nintendo said "No, they're not; Sylvia's similarities are all coincidental, and her shared Holy Blood is from some other distant relative of Claude's!" Difference is, Claude/Sylvia being siblings is vaguely irrelevant in the grand scheme of things, so its more a "Ok, you wasted all that time implying she was, only to say she's not?" Granted, $5 says they INTENDED Them to be long lost siblings, but then they recognized "wait, we're strongly promoting incest cause those two CAN mate..." and decided to go against it later <_<