If it makes you feel any better, my sister is actually quite a lot worse than this -- and I find it really embarrassing. She's 27 and has never had to pay her own way in life at all. My mothers always been very generous and supportive of us both, but largely that's been because she wanted us to be able to get ourselves into a position where we'd have good jobs and be able to support ourselves. In my case, my mother supported me through uni (and helped loads financially through many years of postgrad) after I had my son at 19 (and was a single mother). I now have a very good job, a lovely other half and I'm expecting another baby, and we pay for everything ourselves. In fact, it now makes me very happy to be able to do stuff like get the bill when we are out somewhere with my mother.
However, my sister is like some kind of leech and, what's worse, is that she feels all entitled about it. Briefly summarised, she's always lived with my mum (or with my stepdad) and has never paid for anything whatsoever. She's started no less than 6 different courses at college/uni and has failed every single one of them (largely because she just doesn't bother putting in the work). This has cost my mother a considerable amount in fees etc. Added to that, while pretending to be a student (and still living at home, completely for free) she ran up considerable debts, which my mother has paid off for her. In between bouts of being a student, my sister has tended to do absolutely nothing unless forced to take a job. She has also refused to sign on during those periods because, well, they'd insist she applied for jobs she thinks are beneath her. To make matters worse, she has chosen to quit a number of the jobs she has taken because she just doesn't like them or she's actually been sacked.
My mother, in despair, sent her out to live with my step-dad (who's in the diplomatic service). So she went out to live with him and did absolutely nothing for 9 months. Eventually she got a job out there, but she gave it up and came back home to live with my mum. She's now considering going out to live with my step-dad again now that he's in a new posting. My step-dad quite likes this, tbh, because he obviously gets quite lonely and likes to have someone to live with etc (even though she is absolutely a nightmare to live with: noisy, inconsiderate, messy, etc, etc).
Recently, one of her friends had a go at her about her lifestyle choices. My sister was really angry that her friend had dared to suggest that, at 27, she might want to live on her own/pay her own way in life etc. We'd all been to a family do the day before and were sitting in a cafe (where she's ordered the most expensive stuff on the menu, as always, and she had to have both a £4 fruit smoothy and a coffee simultaneously ) and she started ranting about it. She actually said (something to the effect of), 'why should I have to get a job I hate and worry every day just to pay the bills?'. The OH and i just looked at each other aghast, and my mum made some kind of comment along the lines of 'sound like your life then?'. Of course, my sister didn't take that as a hint. She also insists that she is currently paying her way because she uses her savings (from the job she recently had, and which she could accumulate because she had no outgoings) for her leisure whims etc. I must (of course) be imagining that housing, utilities, food etc all still need to be paid for (and aren't cheap).
The worst thing about it all is that ever since my parents split up (and we're talking about 18 years here) my mum has worked unbelievably hard (2 and 3 jobs) to make ends meet and so that my sister and I could have the kinds of opportunities she didn't when she was growing up. My mum is now quite well off, but only because she works so much. I have a horrible feeling that my sister will still feel that it's OK to sponge off my mum and step-dad when they retire in a few years too. It all makes me really angry tbh, but it's hard to get my mum to see that it would be in everyone's best interests to just throw my sister out to fend for herself.
So, anyway, I thought I'd share. I'm sure someone out there has a sister even worse than mine too.