Muchas gracias!

It could mean: I have washed all the shirts for all of his/her brothers. I understand it like the person who did the washing, did it to each brother, assuming that he/she went to each brother's house. Or he/she did the washing for the shirts that belongs to all of his/her brothers. It's not very clear the way it's written. And like Maisha said, it could be translated in a different way.

Hi! This means "I cleaned all his shirts to his brothers" It is not fully correct. If you want to say "I cleaned all the shirts of his brothers" then it would be "Yo lavé todas las camisas de sus hermanos". Hope that helps! :)