Am I committing a sin by saying "Hasbi Allah wa ni'ma al-wakeel" (Allah is sufficient for me, and He is the best Disposer of affairs) against my sister after an argument, knowing that it is not permissible to supplicate against family members, and what should I do?

"Hasbunallah wa Ni'mal Wakeel" (Allah suffices us, and He is the best Disposer of affairs) is a statement of entrustment and seeking refuge with Allah Almighty, asking Him to suffice the speaker in what concerns them. As for "Ni'mal Wakeel" (the best Disposer of affairs), its meaning is a praise for the One who manages our affairs and is our Guarantor. This remembrance is not a prayer for victory, but rather a prayer of seeking refuge and protection with Allah from worry, fear, distress, or calamity. However, people have started using it in disputes, intending it as a prayer against their adversary, similar to saying "Allahu Akbar alayk" (Allah is greater against you), which turns it into a supplication against the addressed person. Sheikh al-Islam Ibn Taymiyyah said that the phrase "Allahu Akbar alayk" is a prayer against an adversary, and if it is not based on truth, then one is being oppressive. One should not supplicate against a sister, as it involves severing ties of kinship, especially if the disputes are trivial. The elder sister should be more forbearing, patient, and keen on her sister's well-being, seeking forgiveness for her, apologizing to her, and starting a new relationship free from quarrels.