Is it permissible for a stepmother to breastfeed her husband's nine-year-old son so that he becomes a brother to her sons and daughters from her previous marriage?

The prohibitive suckling is that which occurs within the first two years of life. As for the suckling of an adult, it does not establish prohibition according to the majority of jurists, and this is the view held by the Mothers of the Believers, contrary to 'Aisha. Based on this, no prohibition is established through suckling between your child and your wife's child from another (man) by him suckling from her at this age.