What do you think about me asking out a girl whose sister I also asked out recently?
What's the reason for that?  Does this girl know that you also like her sister?
Yes, I think she knows my secret plan, so it's time for a new one
You shouldn't be dating someone whose sister you were also interested in in the past.  Both sisters will know what is happening, and they will feel awkward whenever they see you.  It's best to move on.
I never said that I liked her when I asked her out. Oh well, it's time for a new plan...
Try to stay away from asking siblings out, that could only get people hurt. Why would you ask someone out if you don't like them? That confuses me too.