. Tell your partner you will only pay 1/4 of the cost of living there from now on, since there are 4 adults who are capable of paying
++++++++++++++++++++++++++++
This ^^^

Also explain to him that it is not normal for adults that age to be living at home if they are not going to school/college AND working part time.

Leave or at the very least separate finances....Do not put up with your money contributing. Divide all bills in half...Pay your half and no more....Or insist on paying 1/4 of the bills for four people.

Stand up for yourself.

Leave for awhile and see if your SO and his spawn will sink or swim. Just for an experiment. I know that sounds like playing games, but it'd be interesting to see if they could make it on their own. If they can then you can decide if you like the single life, or invite SO to live with you

My friend was in this exact situation and she and her DH helped the 2 SSs (similar ages) set up in an apartment. They made a plan with them of how they would afford rent and set a date on when they would move out. My friend says it is the best thing they ever did.

It's time for some new rules at your house! This is how I forced my SD 22 out. She was working 2 nights a week at a bar after failing college. She partied the rest of the time, was lazy, made messes in the kitchen & left me with all the dirty dishes to wash. I turned into super bitch. I sat her down when DH wasn't home & explained that it's time act like a grown up not just say you are one, get a full time job, clean up after yourself & pay your own way. I didn't expect any change & of course there wasn't any until she saw I was serious.

The first thing I did was to start throwing out anything of hers she left out. Make up all over the bathroom sink, in the garbage. Wet clothes on the bathroom floor, in the garbage. I just walked around the house with a garbage bag & loaded it up. Dirty, baked on batter on dishs from making cupcakes at 2 am? In her bed! That's right I loaded up all the dirty, sticky pans & dishes & put them on her bed. She thought I went crazy. DH wasn't too happy either but I was over his allowing her to do whatever she wanted.

I called the cell phone company & put her on her own plan with the bill addressed to her that for once she had to pay. I felt like I was at war but this was my house & I'll be damned if this little twit was going to run me out! It worked! She moved in with a friend & eventually in with my oldest who rents a house. Now she complains about how messy her SB is. I say, yup it sucks huh, karma is a bitch! We have a much better relationship now too.

Do NOT put up with this another day! I used walk around my own home angry all the time or I'd become depressed because it's not your home anymore, it's like being held hostage. I say take make your power girl! You have more than you think! If DH gets mad so be it, how long have you walked around mad? Are his feelings more important than yours or those 2 brats?

Once she was out it was just the 2 of us & we are pretty darn happy. We do what we want, wherever in the house we want & life is not revolving around a step brat. His son is still a threat & came back for a week this summer when him & his GF were on the outs but he soon found out that it was not going to be the free ride he had always enjoyed. I had a list of chores for him to do everyday, woke him up early, had a password on my laptop so if he wanted to stay up all night surfing the web he'd have to go to the cold basement on his dads. I was not all warm & fuzzy, way more business like & after a week he was gone, thank God. I love my step kids, I've known them since they were 3 & 4 but I don't parent the way my husband does, I raised my 3 boys to be strong & independent & thank God, they are. I don't mind helping any of them but help does not include enabling.

Bravo no-win. Well done.