Is there a reason Robyn is excluded from most adaptions?

[neatoman]

Remember April's sister Robyn? Guess how many adaptations she's in? One, she shows up in the 2003 show and that's it. She's not in the the Fred Wolf cartoon, the 2012 show, the PD movies or Rise.

Is there a reason for that? The closest I think we got was April getting a time travel duplicate and mentioning she always wanted a sister.