Have I become sinful by rejecting the suitor who proposed to my sister, despite her approval and desire after performing Salatul-Istikharah? And do you have any advice for me and my sister?

A girl's marriage should not be delayed if a suitable suitor proposes, based on the Prophet's (peace be upon him) saying: "Three things should not be delayed: prayer when its time comes, a funeral when it is ready, and a previously married woman (or a virgin) when a suitable match is found for her." If the sister expresses her desire for a suitable suitor, she should not be prevented unless the lack of suitability would bring shame to the family according to custom, such as the suitor being an uneducated driver while she is a teacher. However, if the prevention is without a legitimate Islamic reason, then it is a sin. The sister should not grieve if the matter eludes her, for it might be better for her, and she should know that provisions are decreed, and "so that you may not grieve over what has escaped you, nor exult over what He has given you."