<human>: Ok, a year or so ago, i had a one night stand with a girl [f22]. 

We didn't talk to each other very much after this, and after a while, I had sex with her younger sister [f20]. 
They are quite close, and I always assumed the younger sister told her about it. (It's now painfully obvious that she hasn't said anything)
I never talked much to any of them after that, except for some casual flirting on snapchat, or if we met at a party.

But a couple of weeks ago this changed, as the older sister and I hooked up after a party, and have been getting closer and closer since then. She wants me to join her to a family gathering, and it looks like she is serious about wanting to be more than fuck buddies.

So my question is this: should i tell her, or is it best to leave it unspoken? I want to see where this relationship goes, but I dont want her to find out on her own  at a later stage either, and make her insecure/sad.

Can you give me a shortened version of the text?
<bot>: Had one night stand with two sisters, now I'm getting close with one of them. Should I tell her about my history with her younger sister?