My pregnant sister`s boyfriend asked me out

My 13 year old sister is preggers and the soon to be daddy doesnt know yet. But he happens to be my x-boyfriend from a while ago (hes not 13 by the way hes 15) and like 2 hrs ago I got a text from him saying he thinks he still likes me and wants to get together sometime...but I dont think he knows that I know hes going out with my sister.
So im like really confused should I just say no? Or should I tell him about my sister? But I really dont want to hurt his feeling because he is a really sweet guy. So how can I tell him no gently?
P.s. My sister told me not to tell anyone shes preggers

just tell him that he is a sweet guy but I've moved on,im sorry

Just don't let him hang.
Tell him you are a nice guy, but I moved on, sorry.
Talk to your sister please! tell her she needs to tell him. Maybe (if they broke up) they can figure out something.

have your sister tell him shes pregnant then see what he says to her

Tell him no. Just one simple, easy word.

Just tell him, thanks for the offer but, no!!
There are plenty of other guys that you can date.

...a sweet guy, huh? One who dates you, then later dates and preggerfies your sister, THEN starts hitting on YOU again. You consider that to be sweet?

she needs to tell your rents

y would you and your sis date the same dude anyway?? well I think you need 2 talk 2 your sister. Tell the guy no sorry you moved on.