I (23M) am not sure how to approach a situation with my sister's best friend (18F).

My sister's friend is usually really outgoing and kind, but I noticed last time she came over that she seemed more reserved around me.

I've met her only a handful of times, but she's always given some signs of interest like asking me questions, proximity, breaking eye contact with me to look down (submission), and being more reserved around me than she normally is with others. Two times when she came over she came up to ask me how I'm doing, but she's friendly and talks to everyone so that doesn't mean anything. I'm not fully certain if she's into me or indifferent about me because I see her maybe 3-4 times a year and my sister won't help give me any info about her.

My sister has told me her friend is not interested in me or not interested in older guys but she giggled when I asked her if her friend was coming over. She told me not to act weird. My sister always acts weird about it and is against me dating her friend because she says I'm too old for her, which is obviously bs, I've dated a 18 year old before and she liked me a lot. My sister won't even talk about her friend at all. I think she just told me her friend is uninterested because she doesn't want me to date her.

I am not sure if I should just follow her friend on instagram and try DMing her to invite her to the beach or something, I would rather be rejected and know she's not interested so I could pursue other women and move on.

Or would it be better to just let things develop naturally and continue escalating if my sister invites her friend over again? It's a bold move to DM her friend because then her friend may be uncomfortable when she comes over if I ever see her again. Personally, I also think DMing her is thirsty and she'll think I'm creepy.