If it was a listening exercise then you need to answer with the version spoken - I would expect that to be ... fy mrawd i'n ..., here, and i'n and yn are not pronounced in the same way.
If it was a translation exercise, Mae fy mrawd yn gweithio mewn shop. should be accepted.
This is one of the big questions about learning Welsh and causes much confusion although the answer is quite simple before nouns.
Mewn = 'In' + 'indefinite article' so since there isn't an 'a' in Welsh, 'mewn' in front of a noun = 'in a'
Yn = 'In' + 'definite article' = "in the'
Mewn siop = In a shop
Yn y siop = In the shop