i hate my situation...

[IaMtHeEaStErBuNnY]

i need some advice, seriously... no jokes, no bull... this is all the truth... in my sophomore year, i met this guy sean while i was playin handball at bedford park... i had asked him for a cigarette lol... we started hanging out once in a while, and then chilled at his house alot, until we became best friends... we were best friends for about a year, and one day i met his sister, i never knew about her because she was living in penn and he never talked about her... so we started talking, but not much... then this year, after me knowing sean for over 2 years, and me knowing her for a bit over 1, we start talking alot... we hung out secretly over the summer, cuz we knew he would have a huge problem with me hanging out with his little sister... (hes 22 now, and shes 16)... so then in november of last year, we realize we really like eachother, and we start to date on the dl... we dont tell him... but my guilty conscience tells me i must... were still together to this day, but he still doesnt know, but i fear if he finds out hes gonna have us break up and he wont ever talk to me again... but i love her so much... what do i do???