Why do you think we never see Charles sister?

This post from a year ago asks "what happened to Charles’ sister?" https://www.reddit.com/r/brooklynninenine/s/gYT7AmDU7U

A lot of people referenced the doll house flashback and then commented that it was likely a cousin. But in Season 2 Episode 2 (LMAO I’m watching it right now), Charles tells Rosa his sister was supposed to go with him to his ex-wife’s engagement party, but she cancelled on him. I feel like this implies that they’re somewhat close, at least close enough for Charles to ask her for that favour initially. Lynn seems like a good father too, so I’m wondering why we don’t see her at the wedding and it feels like Charles is Lynn’s only kid whenever they interact. I’m thinking since we know Charles’ parents fought often and divorced eventually, maybe his mom took custody over his sister and his dad took him. Charles does not seem as close with his mom as she never appears on the show… despite the fact that when Jake and Terry fight in the pit he tells them it stresses him out because it reminds him of when his dad fought with "(his) best friend: (his) mom." Probably meant best friend in past tense, back when he was a kid. He mentioned hanging out with both his parents and them having to be in separate rooms, so I feel like Charles’ mom probably limited his sister being able to see their dad, so eventually they became estranged. Or she’s his half-sister from a previous marriage before his mom married Lynn Boyle.

Anyone else have any fun theories 😂?