<human>: Can you give me the gist of the text in a nutshell?

So my best friend has a little sister who's only a year younger than me (I'm 21 she's 20 next week). This summer I've been back from college and she's been hanging out with our group of friends a lot more and we've gotten quite flirty. I was going to ask her out but I'm worried it will piss off her brother (he already gets annoyed when he catches us texting or snapchatting each other) is there a way I can avoid the risk of ruining a friendship? Also how do I bring it up to her that I actually like her and its not just a jokey flirtation anymore? 

Thanks in advance and sorry to any mods if I've broken a rule I'm new here
<bot>: like my best mates little sister, how do I avoid pissing him off? How do I bring it up to her?