S.E.
Gee, sounds like typical sibling behavior and typical teen age behavior.
If the kids won't do their chores and pick up after themselves, they need consequences and need to lose priviledges.
The 15 year old is too old for a babysitter, but the younger three aren't. It might be in your best interest to go ahead, finances permitting, to have someone come in and do occasional babysitting and light house keeping.
I'd have a sit down with the older two - as they will set the tone for the younger two and get them on your side and to be a united front in some areas - such as chores. And put some rules around their interaction with one another.
The 7 year old probably idolizes the 15 year old and just wants to hang out etc. If the 15 year old could structure some some for his little cousin, that will make a big difference. Like a guys day where they can go to the park and play ball or to the mall or something like that. And the 7 year old agrees to give the 15 year old his space otherwise.
Hubby needs to back you up on this.
What a lucky young man to have an aunt like you who was willing to take him in and be a second mom.