Why is my close friend hiding her younger sister from me?

i've been friends with this girl for 2 years , we would of first meet summer 2024 and never once has she or her girlfriend's mentioned that she has a younger sister. i've never seen them together and isn't even a picture of them on her social media at least pictures that i could view.

i found out that she had a sister after a read a news paper article about her mother and an obituary online that clear states there is 2 sisters and gives the younger sisters name. i looked online and could not find any accounts on sites facebook or instagram but is a tiktok page in her name and comments are from local accounts so definitely her

on the weekend i actually meet her inperson , it was weird i went into this chain restaurant and she was the hostess, i recognised her rate away as the same girl from tiktok page i saw. it also makes sense that she works at that place as her cousin in one of the bartenders. she didn't mention that she knew who i was or that i knew her older sister and i didn't say anything either but it was definitely the secret little sister.

now i don't know what to do , should i just ask the girl i'm friends with openly if she has a sister and see what she says? does she already know that i meet the sister at the restaurant so like secret isn't a secret anymore right?

Updates
11 d
i talked to her at her work , at first i sensed she was annoyed at me but then once i started to talk to her she seemed fine and we got along really well for a couple minutes but was at her work so could not talk to her forever. no mention of secret sister at all though and still not sure why
Why is my close friend hiding her younger sister from me?
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