This sounds scarily like my older brother in a lot of ways - he's the definition of an irresponsible fuckwit tbh. And he has been totally enabled by my mum.
Unlike your (D)SS, he did move out fairly young - because he'd managed to get a woman with a flat pregnant (lied about his age and claimed to be several years older, they worked together so she didn't know) so sponged off her for a few years. She kicked him out, he came back to mummy. Stuck around for a couple of years, moved in with the next older woman with flat (and children), she put up with him for a few years then kicked him out, back to mummy...rinse and repeat at least another 4 times with different women and flats. One memorable return was when I was about 10 and ended up having to share a bed with my mum for months because he was using my room - he was eventually thrown out that time because he came home pissed as a fart and got violent and aggressive to the extent we had to barricade ourselves in the room but that was only after months of me tiptoeing around him. Younger siblings are not immune.
I'm quite a bit younger - 15 years - so when I moved out of home at 17 he was in his early 30s, and offered to move in with me (I had secured a 2 bed council place) where he promptly refused to pay council tax, or bills except a contribution to rent because he'd been (fraudulently) claiming HB and CTB at his previous flat and didn't see why he should be worse off. He'd been living with me for a couple of months when he started bringing people home to party when I was working the next day and leaving abusive letters. After I stood up to him he came home, pissed as a fart, and started threatening my then boyfriend. I told him the next day he'd have to leave, he started yelling abuse and shouting about squatters rights. I got a locksmith out that night when he was at work.
He went back to mummy. Again.
Since then I think he's moved in and out of my mum's at least 5 times - he's now in his mid 40s and his latest trick is getting jobs abroad, moving to far flung places for commission only roles, staying a few months and coming back utterly skint...and moving back in with Mummy, who is now a pensioner who he scrounges off.
He's had more jobs than I've had hot dinners, and if he wrote out his CV accurately I reckon it would rival the King James Bible in length. Every job he gets is "the one" and "going to make him a millionaire" (normally dodgy sales roles...he tried to sell my mum who was on benefits a Kirby vacuum for £2k), and every time he loses one he's so hard done by. He also knows everything about everything - I work in a professional role in financial services, have done for years, have a degree and postgrad qualifications. He worked in a sales roles overseas for 3 months and came home lecturing about how I know nothing about economics. He's been a chef (not), a computer wizard (not), a stockbroker (salesman), commodities trader (salesman)...
He got himself deported from Indonesia for overstaying his visa (but it wasn't his fault, it was his mean boss and and and)
He's currently overseas again but talking about coming back. He threw a strop at my mum the other day because she didn't answer when he called her - she was on the phone to someone else and when she explained this he said "he should be top priority."
He's also a sexist knobber who thinks women should serve him, believes in the man as the head of the family, and so on.
The point of all of that...this is your future if you do not stand up to him NOW and make him grow up. My mum has enabled this crap for decades, and now finds herself as a pensioner stuck in a small flat with a boomerang son who moves back, runs up debts, eats and drinks her out of house and home and treats her like crap. She's had numerous opportunities to make a stand but she never has.