You're not wrong at all, especially if your b-in-law is 27 and making enough money to support an apartment.
I have a feeling there is more to the family dynamic than what you've said ie: why your husband is so protective of your brother and feels the need to be a parent to him. Is there some sort of disability he is dealing with?
If it were me, I would talk with my husband and have him consider how him living with you all is hindering him from growing up and being a man of his own. Your bil deserves the opportunity to grow up even though it might feel like a struggle. You cant take care of him his whole life. He deserves the opportunity to live and have relationships and possibly a family of his own. I agree with different posts that it's time to boot him out of the nest. Give him something like 90 days notice to find a place and and a roommate and in the meantime give him more responsibility. He should be paying 1/3 of the expenses - rent utilities cable etc. If your husband is committed to having him stay then you need to move to a bigger apartment or house where you all can have your space.
I hope this works out for you. I cant imagine how you have lasted this long.