Drinking one’s wife’s milk has no effect and does not create the relationship of mahram because the breastfeeding which has an effect is that which consists of five feedings or more within the first two years, before weaning. Therefore, if one’s wife has milk in her breasts and it is feared that by taking them in the mouth, milk will also enter, then it will be impermissible to suck on the breasts. Otherwise, it will be permissible.

And Allah knows best.

Sources:

• Daruliftaa
• IslamQA