I feel like sharing my takes on the missing Rise siblings.
In the comic Splinter said there was a sister and a brother. But hear me out. He was wrong about the brother because said „brother“ is actually a trans girl! I just feel it in my bones, both missing siblings are sisters!
So fist, Frida aka Big Mamas assistant. She’s rather stoic and seems cold at first but much like her brothers she’s actually very loud, she was just trained not to show it. Speaking of loud. She’s not loud in the vocal sense. She uses sign language a lot. She’s loud in a more figurative sense. Once she comes home, she’s learning to unmask be herself. Also she’s technically Mikey’s „twin“ and the youngest now. While Frida doesn’t really care about who’s older, Mikey takes pride in not being the baby anymore.
Next is Venus. Yes, Venus. As I mentioned before she’s trans, having been stuck in a different dimension she has no idea about earth and the norms. (The Dimension part was mentioned somewhere I think.) She also didn’t go by Venus at first. She went by Slash when they met. She chose the name with Mikey’s help since she wanted to match her siblings. She’s the biggest and technically the oldest, being Raphs twin. But Raph still acts like the eldest because he is used to the role, but Venus is learning to be a big sister to the others, she and Raph act more like equals and don’t really care for who is older.