"Say, ‘I do not ask you any reward for it except the love of [my] relatives.’" (Qur'an 42:23)
Thank you for your question. According to Zamakhshari in his commentary al-Kashshaf, using the pronoun fi gives the meaning of the Family of the Prophet (saw) being the place of established love, whereas the pronoun li only would explain who the love was for. Fi therefore gives a greater meaning.
May you always be successful