Wow, she really does sound like a proper cow I'm afraid.
I think you need to change tack a little. You need to get the claws out yourself a bit. Like very much CheerMum's suggestion that she reminds you of one of those 'stupid little dogs that humps peoples legs to get attention'. Wouldn't do any harm for you to word it as 'God, you know you remind US of one of those stupid little dogs that humps peoples legs to get attention! Don't you think you're getting a bit old for all this, hmm?' Looking as if you're about to burst out laughing, but trying frantically to hold it in (much more effective than a forced out-loud laugh).
Next time she makes a nasty comment: 'Wow, you really are getting unpleasant in your old age, X. Don't turn into a bitter old spinster, for ALL our sakes!' - with a smile.
She is immature, basically. She doesn't like the idea that Big Sis has things she doesn't, even if she doesn't want them - but, if she's got a problem with holding down a good relationship, she probably does want them - or at least is afraid that she isn't going to get them. If that makes sense!
This is something to sympathise with, but as she's being so nasty to you - gloves are off. Hit her where it hurts - point out her obvious jealousy and acting-upness, and laugh at it. She will HATE it, and it will be the most effective way to stop her in her tracks.
Oh, and don't waste time feeling inferior/less attractive etc. She acts like this with you because she needs to - if you weren't a threat, or someone she felt she had to 'keep on top of' - she'd probably either ignore you or be your best friend.
No harm overall though in spending less time with her, and telling your DH in detail why. You need to make it clear to him that her behaviour is well on its way to ruining your relationship with her, so if he does think she's 'harmless' - he'll listen to his wife and keep her at arms' length for her own good, before she messes up her family ties.