Unsure of how to talk with my sister, 16 year age gap

To add context, Im a 26M and have 2 other siblings, 4 years between each of us, all of us boys. We grew up in an emotionally repressive household, so me and my 22M brother dont express ourselves that much and are pretty calm and laid back, while my 18M brother is loud and expressive in a hypermasculine way. My sister was born when I was 16 and I am her main father figure as well as her older brother, so she hangs around me a lot and is extremely attached to me.

My 10F sister talks normally about 50% of the time, which is hard for me to manage but I make do. Its the other 50% of the time that takes me to my wits end. She is super full of energy, and she starts talking super cutesy or acts like a character she watched on Youtube or in a show, a caricature, and it drives me insane. Probably an inherited social judgement due to my upbringing, but it infuriates me when she starts talking like that (think Valley Girl + little baby + swapping Rs for Ws), but I dont tell her to stop because Id rather her be expressive and feel comfortable doing it around me than me scolding her because I grew up unexpressive and have fared decently since then.

My reaction is probably a combination of the age gap, my upbringing and the social yes-nos imposed on me, and maybe some level of immaturity from me for not being able to accept it and live through her childhood. I want her to feel safe and be able to express her since I know it was not a good thing that I was unable to as a kid, but I also do think she goes too far and am afraid that if I tell her to tone it down she would flip into a repressed state like me, since my family has not changed and is still emotionally repressive, they just dont impose it as harshly since both my parents are old and kinda leave the parenting to me. If I, as her only current unconditional expressive outlet, turn into a space that she gets reprimanded, however light, Im afraid that would take this safe space away from her. Shes also started developing attachment issues and is really sensitive to people mocking her or getting upset with her, so I have to keep a plain or positive face whenever Im around her so as not to further her anxiety issues. I dont want to be overprotective, but I feel like I have no choice since our family environment is foundationally hostile and oppressive.

I was wondering if anyone has any strategies to help me navigate or if what Im doing is right and evens out the family dynamic. Im fine dealing with the instinctual frustration, Ill chalk it up to a fault of my own. Im just worried it might socialize her wrong and she may lose friends or get bullied if she keeps habits like these as she gets older. She may grow out of the baby talk and stuff, but the sensitivity and all that is hard for the to tread around and to try and nudge her into a 'people will upset you sometimes and thats ok, and people will get upset with you and thats ok'.

TL;DR: My 10F sister is extremely sensitive, might have attachment issues, and talks like a baby so she can endear herself to others, and I, 26M brother acting as the parent figure, am unsure how to approach it and try and help her navigate these emotions without being oppressive since our family is already prone to hostility and reprimanding.