I can't say for sure as it's not explicitly clarified in the game, but it is likely that Marcella was in fact an older sibling and not sister ! The consistent referral to Marchie as the neutral "sibling" and the heavy use of they/them pronouns implies a nonbinary character, and I think the love and appreciation the bunnies have for their older sibling is portrayed very well !!
I get you on the clock puzzle, it took a hot minute for me to find exactly how to solve it