He does not suspect you of making trouble, and he will not think about you in any way if you let him alone and keep quiet.

When he does offer himself you will know it; at least your sister will tell you if she has accepted him.

If she refuses him point blank, you will have nothing to do but to keep her steady.

If you see her hesitating, you must break in at any cost, and use all your influence to stop her.

Be bold, then, and do your best.