Clearly an oversight, as 'brought up' was accepted in other sentences with 'wychowany'. Added.
Hi Jellei, I have just finished the Polish Tree - Thanks for all your help (& also immery). I had so much fun, and have begun to remember some of my polish. I look forward to visiting the site often.
Couldn´t it be: "He was raised by his own grandma?" "own" would emphasize the reflexivity of the pronoun "swoja".
We have an equivalent of 'own', and it's "własny" (and its forms), so while 'own' may be accepted somewhere, I think that generally there's no need to accept it. It adds a word.
You could say "przez swoją własną babcię", it would be correct, but of course 'własną' is more redundant than in English, as 'swoją' already says that it's his own and not some other guy's.
We could say "brought up by..." in English, but I don't think its what the exercise is looking for.