Is it permissible to feed sixty poor people as expiation for saying "you are forbidden to me as my sister is forbidden to me," instead of fasting for two months, especially given the questioner's profession as a dentist, and can the feeding be done by distributing ready-made rice meals?

What you did constitutes zhihar, because whoever declares his wife unlawful to him, likening her to someone permanently forbidden to him—such as a sister—is considered a muzahir (one who commits zhihar). This applies whether the zhihar was explicit or implicit. You must perform the kaffarah (expiation) for zhihar in the prescribed order: emancipating a slave; if you cannot find one, then fasting for two consecutive months; if you are unable to do that, then feeding sixty needy persons. The obligation of this order is not waived merely by ordinary difficulty. We caution you about the necessity of guarding your tongue and repenting to Allah for this statement, which Allah described as "reprehensible and false."