Il try an summarise my issue.

im step dad. My partner has 4 kids. 
We have been living together for 8 years.

her eldest is 21, who has moved her partner in who is 26 semi-permanently about 18months ago. Eating everything In Sight, leaving mess everywhere they go, using all the towels, filling the kitchen of dirty dishes, and even complaining if there favourite food isn't there.
they are both working, but neither chip in towards household costs.