✨It’s a little lengthy I’m sorry!✨

I have some really close family friends who I’ve known for almost 10 years, especially the youngest in the family who is 4 years younger than me and has always been like a little sister to me.

One of her brothers is my age and truth is I’ve always had a thing for him ever since I met him, not to mention we’ve been flirting on and off for all these years but have never actually done anything with each other. We’re at a stage right now (first time since we were young) where we’re both single and our sexual tension is growing really strong which is leading into talking about having sex with each other asap

His little sister is now half way through her teens so she would be able to understand these things now, so I’m starting to feel guilty about the fact that I’ve been flirting with her brother behind her back for all these years. And especially feeling guilty now that the tension between me and the brother is insaneeee

I originally was against the idea of having sex with the brother for the sake of the sister (betraying her) and the family (what they would think of me) if they found out but now I’m starting to cave and I really want to do it

Give me some opinions/advice please!

Should I stick to my morals and stay away from the boy or should I just go for it?

Am I right to be thinking of the sister or am I just overthinking it and it’s normal given the situation (constant flirtation ever since we met almost 10 years ago) between me and the boy??

Edit: I just want to add that I have high morals and am not the type to even go near friends exes, siblings, etc. so I’m struggling to figure out if going for the brother is a good idea or not