Is it ok to ask out the sister of someone who rejected you?NOT TO GET BACK AT HER!!!?

in the beginning of the school year i met 2 twins (makes matters worse) and i asked the one with the longer hair out after a while,she rejected me but we didn't get to know eachother enough and that was understood.After 4 months i got to know her sister ALOT and i sorta started to like her too.and i'm sure she likes me the signs are everywhere. laughs at all of my jokes even ones i don't think are funny,she's ALWAYS smiling at me,she even turns really scarlet rose red when i talk to her 30% of the time. so please help how would you feel if you were asked out after the same guy/girl asked out your sister,or if you were just asked out and he went to your sister/brother next, what if she thinks i just want to use her to replace the other!?