How can I overcome my remorse after preventing my sister from marrying a man who proposed to her, which led to her missing this opportunity?

If you described the suitor with qualities he does not possess, then you have sinned in two ways: First, slander (buhtan), which is a major sin and necessitates sincere repentance to Allah, accompanied by remorse and a firm resolve not to return to it. Second, breaching the trust in advising your sister, for the one consulted is entrusted. Therefore, you must seek your sister's forgiveness and ask her pardon.

However, if you mentioned his negative qualities to warn your sister, then there is no sin in that. It is permissible to mention the suitor's true characteristics when consulted, as this falls under sincere advice (nush) and is not among the forbidden backbiting (gheebah).