As in our society the practice of adoption is only with blood relatives due to the concept of Mehrum: and what it means by Dr shabbir's translation "And if you do not know who their fathers were, then they are your brothers,"
call with their father's name and when you dont know who their fathers were they are your brothers, sisters ....
but when we say name your adopted childern after their fathers, (which seems logical though, but how we get connected to) and if you do not know who their fathers were, then they are your brothers, sisters .....
Generally Accepted Translations of the Meaning of 33:5
It would still be better if those whom out of affection, you refer to as your sons, are called with reference to their fathers. This is the better way according to Allah's Laws. However, if you do not know the names of their fathers (like when you find a lost child and do not know whose it is) then such children are your brothers in Deen and your friends. (As mentioned afore) anything that is said inadvertently, like if you have earlier pronounced someone as a son, no cognizance would be taken of that. Cognizance is, however, taken of all actions which are undertaken intentionally and with full consciousness. Remember! Adequate provisions have been made in the Divine Laws for not taking action on inadvertent matters, so that such things would not hinder your process of development.
Name your adopted children after their fathers; that is more just in the sight of God. And if you do not know who their fathers were, then they are your brothers, sisters and friends in faith. You shall treat them as family members. There is no blame on you for an unintentional mistake. What counts is the intention of your hearts, for God is Forgiving, Merciful.
Dr Shabbir Ahmed
[As for your adopted children,] call them by their [real] fathers’ names: this is more equitable in the sight of God; and if you know not who their fathers were, [call them] your brethren in faith and your friends. However, you will incur no sin if you err in this respect: [what really matters is] but what your hearts intend - for God is indeed much-forgiving, a dispenser of grace!
Proclaim their real parentage. That will be more equitable in the sight of Allah. And if ye know not their fathers, then (they are) your brethren in the faith, and your clients. And there is no sin for you in the mistakes that ye make unintentionally, but what your hearts purpose (that will be a sin for you). Allah is ever Forgiving, Merciful.
M. M. Pickthall