Tell your best friend your dating his sister their dating magazine
Make sure the sister actually returns your affection.
If there is no chemistry between the two of you, and you have never even talked to her, perhaps you should get to know her first before you ask her out on a date, so as to keep yourself from an awkward situation if she were to turn you down. Reassure her that you have talked to her sibling first, and that you just want to get to know her better on your own.
Yet we cannot help who we become interested in and who we want to date.
When you find yourself falling for your friend's sister, there are some rules you should abide by in order to maintain a good relationship with your friend, as well as her sister.
For some context, I met her around a year ago and have gradually developed a pretty severe crush on her.
At first, It was just an infatuation, but as I've gotten to know her better I've developed deeper feelings.
If so, I don't think there would be a problem if you're honest with him.
Explain your feelings and make him believe you like her a lot and don't just want to get in her pants.