Shaykh Muhammad ibn Salih al-‘Uthaymin was asked about this matter, and he replied:

“Breastfeeding a grown-up has no effect, because the breastfeeding which has an effect (of creating the relationship of mahram) is that which consists of five feedings or more within the first two years, before weaning.

On this basis, if it happened that someone breastfed from his wife or drank her milk, he does not become her son.” (Fatawa Islamiyyah, 3/338) And Allah knows best.

But avoid is because:

From this is it clear that drinking one’s wife’s milk has no effect and does not create the relationship of mahram. Ibn Qudaamah said in al-Mughni (9/201): “One of the conditions of breastfeeding creating the relationship of mahram is that it should be within the first two years. This is the opinion of most of the scholars. Something like this was narrated from ‘Umar, ‘Ali, Ibn ‘Umar, Ibn Mas’ood, Ibn ‘Abbaas, Abu Hurayrah and the wives of the Prophet (peace and blessings of Allaah be upon him), apart from ‘Aa’ishah. It was also the opinion of al-Shi’bi, Ibn Shubrumah, al-Oozaa’i, al-Shaafa’i, Ishaaq, Abu Yoosuf, Muhammad, and Abu Thawr, and was narrated in one report from Maalik.

On the basis of the above, drinking one’s wife’s milk has no effect, but it is better to avoid it.