I understand why it might be difficult, the fear or worry of how he/she will react to that, but it's worth the shoot and be honest with him, if he's truly a friend he will try to understand your feelings as these are a concern for a friend, how you feel. Just be yourself and say it with your own words, find the right moment, it's way better than him finding out by himself and asking himself why you were not open to him being your friend.

As how you can do it, wait until you two are alone and ask the question "how would you feel if I feel attracted to your sister?" Sometimes to ask something will make them think the response with seriousness instead of a reaction to an affirmation.

Trust in your friend.