I'm not so sure it's an elephant as much as a mammoth. I too have referred - at least 4 times I think - during the course of this almost a year old thread to the OP leaving her DH (or that should probably be H). And agree that if the father was objective and fair and treated both his children equally (with age appropriate allowances of course) then it probably wouldn't matter too much if SS continued to be a nasty little bully .... in fact, he'd be very stupid to do so if he had the sort of father who took no nonsense and disciplined as and when required because any attempt at nastiness would be immediately squashed before any damage was done to DD and SS would come off worse. If the father was on the ball and fair he'd acknowledge what his son was like and he'd keep a beady eye on what was going on - hence OP wouldn't have to feel frightened about allowing H to take both his son and daughter without her as she'd be able to trust him to do the right thing.
So yes - totally agree this is mainly an H problem as opposed to the SS per se. Having said that, although he is young, I don't think a 14/15 year old should be absolved of all personal responsibility when it comes to behaving appropriately towards a much smaller and younger child. Quite obviously he does it now because his bloody father lets him get away with it without a murmur, and quite possibly gets a kick out of the "power" this gives him, as well as reinforcing (each time he's nasty and there are no repercussions) that he's daddy's "favourite". However .... I'd question if he does behave the same way to all other younger kids he comes across - like in school, or at activities ? I bet he doesn't, because a) he knows it's wrong and b) it wouldn't be stood for. In that case, he makes a conscious choice when he's foul to DD and I personally think he should bear some of the responsibility for that regardless of the fact his pathetic father condones it. In other words, jut because you can get away with doing something wrong, doesn't make it right. So I quite understand why the OP feels so resentful and hostile towards the SS.
Though remember she hasn't banned him from the house, and, she has said that she's willing to bring DD along on these days out herself - but the boy has vetoed that .... which of course, shouldn't bloody well be his call.
I've said before that I think now - in view of the length of time this has been going on - and the H's entrenched position, that the OP hasn't got a chance he's going to come round and parent his older child properly. He's not even parenting the younger one properly as he's continually shit-stirring - goading the little girl with tales of fancy days out and painting mummy as the bad guy. It must make the DD feel very stressed and torn between both her parents and he's putting her in a completely unfair and unacceptable position. What sort of decent parent treats their children so differently - and what sort of decent parent badmouths the other parent to a child (irrespective of whether they're justified in feeling angry) ?
I feel very strongly the OP should start planning a split - for her daughter's sake as well as her own. I fear she could easily end up having a nervous breakdown with the stress of this situation and the appalling way her H is treating her. However, regardless of what us lot of strangers think, actually splitting from someone is rarely simple and these days, with the cost of living and benefit cuts etc there's no escaping the fact that it's not an easy thing to embark upon and many women in the OP's position would have to make long term plans before they could actually do the deed. Whether we think that's right or wrong, until the OP actually leaves him - or tells him to piss off - then good god of course she has to protect her daughter because the child's own father is absolutely blind to any harm she may come to if it involves the untouchable older child. I suspect also that at the back of the OP's mind is how things would pan out if they did split and DD had contact with her dad ..... clearly, he couldn't be trusted to see the two children separately, and therefore, there's a significant risk that DD may be hurt again. It is possible to arrange supervised contact, but again, this isn't something you can just conjure up at will and the OP would need to take legal advice (IMO) in regard to that particular issue before feeling confident enough to split - when she'd be less able to protect DD from seeing SS than she is now.