Does accusing one's brother of doing "wrong things" with a foreign girl, with the intention of accusing him of fornication, constitute an indirect accusation (qadhf bi al-kinayah) that necessitates a Sharia-prescribed Hadd penalty?

Regarding the matter of slander (qadhf): If the intention behind it is doubtful, the basic principle is the acquittal of responsibility, and one should not pay attention to such doubts. However, if it is highly probable that the intention was to accuse him of committing indecency (fahisha), then this is indeed slander for which repentance is obligatory. It is not necessary to inform him; rather, it suffices to pray for his well-being.