Jealousy is a seriously unattractive (and potentially dangerous) quality. So is the underdog mentality. You're setting your daughter up to fail before she's even begun.
Even worse, jealous of your own sister and niece? Yikes! You should be so proud of their achievements. And I say achievements because, yes, while some people are gifted in certain areas, and some, very fortunate others, do have things handed to them, for the majority it takes a lot of bloody hard work, discipline, strain, pain and sacrifice (physical, mental, financial etc) to attain, and sometimes maintain, the types of accolades you mention. A win for any of my siblings is a win for us all in my eyes.
In response to the question at the end of your original post, the first thing I would suggest is fostering an attitude of gratitude - count your blessings! You have your husband, a healthy daughter, sight, ability to walk, job, education, a home, access to free healthcare, whatever you have - be grateful for it. Your daughter is blessed with amazing traits and abilities too. There are far worse things you and she could be dealing with right now than some, actually, decent A level results, feeling a bit of an ugly duckling, having only one friend and not knowing what to do with herself at 18yrs old. If you really believe she has 'everything stacked against her' you need to broaden your awareness of the world we live in. Do you realise that in some countries, children with your daughter’s types of conditions are being dumped at orphanages and mental institutions, rejected by their families and communities and never given the love they need, let alone the opportunity for an education or to work? Have a read about how women in some parts of the world are forced to fight for their rights, or for basic amenities and resources that we take for granted!
The second thing I would suggest is looking inward instead of at others. Does your jealousy stem from insecurity? Do you feel you have underachieved? Or are untalented? Or unattractive? And not in comparison to anyone else, but in your own self? Be very aware that those insecurities and any bitterness can often be sensed by other people. And conversations or even little comments in front of your daughter over the years, comparing your sister’s mortgage-free, seemingly great lifestyle to your own, will definitely have had some impact on her. If you have a defeatist mentality your daughter might end up the same way. Teaching her, however indirectly it may be, that others are successful simply because they are lucky and that you are unsuccessful because you are unlucky is doing her a huge disservice. I do believe a bit of healthy competition can be good in some circumstances, but we should really be focusing on ourselves and finding our own place in the world, celebrating our individuality and creating our own opportunities for improvement, rather than trying to beat others at their game. It’s never too late to ‘get a life’ and up your own game however. Is there something you've always wanted to do but have held back out of fear? Now might be the time to try. And your daughter, whether she shows it or not, will admire you for anything you (and your husband) achieve. You are who she sees everyday: your activities, your attitudes, your behaviours. Focus on being the strong, positive, role-model for her (and for your niece) that you would have wanted.
Thirdly, I think it would benefit both you and your daughter to get out in nature a bit. Get a map and find big local parks, beaches, nature reserves or nearby national parks you can take some long walks in at the weekends or whenever you have time off. Fresh air and casual exercise can work wonders at very little financial cost. And make for wonderful opportunities to really talk and to bond. Talk to her as a woman, as if you’re friends. Tell her about your childhood, your ambitions and dreams growing up, who you fancied, what you do at work, a faraway country you've always wanted to visit, anything. And you can be honest about things that didn’t go well or times that you may have failed, but leave the self-pity at home.
Gratitude, self-reflection, fresh air...
Good luck!