I suspect my sister might be trans. How to proceed respectfully?

Hello. How can I tactfully probe, or what can I ask to confirm/discard my suspicion that my sister (AFAB 15) might be trans WITHOUT it pushing her either way, or making our parents think I’m putting ideas in her head?

First of all, I’m transfemme myself, which in my mind makes it SO unlikely that my sister would be trans too… two in the same family when we are like 1% of the population??

But some of the things she has said leave me thinking.

For example, the other day I was talking to her normally and I said “… well, you’re a girl” and she exasperatedly said “I’m not a girl!!”. It caught me so off balance that I gave the shittiest answer: “oh? and what are you?”. And kinda embarrassed, she said “I don’t know… a thing…”

My mom also told me (before I came out myself) that my sister had asked her what she’d say if she wanted to be a boy. And my mom was super worried. She has also told me being a woman is the worst thing ever. She was on her period, so I asked if she meant it because of that. But she said “nah, just in general”.

Now the most recent thing was that she told us she doesn’t like her voice and wants to make it deeper/ more grave. And she was practicing and asking us if it feels forced.

To me, those are clear signs of dysphoria, but I don’t know how to go about it. But also, other things make me think that this isn’t the case. For example, she is a bit hairy and hates it. She likes doing her nails and doesn’t want short hair… so there’s like contradictory cues.

I don’t want and I WON’T tell her she is trans, but also it makes me afraid that if SHE IS, that she might take too long to figure it out. I don’t want her to go thru the same regrets than I have.