Really, don't compare yourself to your SIL, I am certain that as she's human, there's a whole bunch of stuff that she's not thri lled about in her life too. If you were happy with your life and situation, you probably wouldn't be looking at her life through rose-tinted specs. So lets forget about her for the moment and think about your life.
I'm going to pick some stuff out of your original post that you don't seem to be happy with, none of it is a judgement from me on your situation - for example I'm happy being a SAHM - I'm just reflecting back the thoughts you've expressed.
You felt that circumstances forced you to give up a job you enjoyed. You feel that you are overweight. Money is tight which means you don't have much disposable income and you've had disagreements with DH about it. You miss adult conversation and feel you've lost touch with your friends. You are feeling tired because of broken night sleep and don't feeling you are getting much family support.
On the positive side, you've become a mum to a great wee girl who you get to spend loads of time with. You aren't keen to leave her in childcare so are lucky you don't have to. You have people you are friendly with at groups. Your DH is wonderful despite the silly arguments.
It seems to me you need to review the work situation, which is at the heart of several of the things that you are unhappy about. You express resentment about losing the old job because it wasn't your plan, what you would have choosen. But you also have recognised some of the benefits of being at home with DD instead of having her in childcare.
You have a few choices here theoretically. 1. You can work fulltime and have DD looked after full time by someone else (Nursery/Childminder/Family/DH) 2. You can work part time and have DD looked after part time. 3. You can work part time hours mainly around your DH's work hours so that there is minimal need for external childcare - this could be from home or out of the home. 4. You could be a SAHM and find other ways to fulfill the social needs that work used to.
What would you choose ideally? Then figure out if there is a way of managing the household income around it. (All the cc cost don't need to come out of your salary)
I suppose I'm wondering - given your reluctance to leave DD - whether actually being a SAHM is actually the best choice for you (if you could get a couple of evenings out a month with adult company) but you are so caught up with the fact that it wasn't the plan, that you were forced into it by circumstances and wouldn't have choosen it before DD that you can't see that you've changed since DD arrived and being a SAHM suits you?
If not, and you are desperate to regain a professional life again, then you have to accept that someone else will need to look after DD whilst you are working! SIL doesn't get the time at home with her DD that you do. You have spent so much effort at looking at what she has that you've assumed that she has it all on top of what you've got rather than recognising that you have things she doesn't. You can see if other employeers could do part-time hours, or higher pay than your last job. Maybe DH would like to have some more time with DD and both work part time?
I can't believe you are jealous of SIL's hangovers... 
it seems like she hasn't really grown up. But if you want baby-free nights out, then you have a DH to look after DD don't you? You could get back in touch with your old workmates and ask if you can tag-along on their next night out, or organise your own. You could ask your mummy friends if any of them fancy a night out. You don't have to wait for someone else to come up with the idea. Other ideas - evening classes to learn something you've always wanted to.
As for your weight. Are you eating healthily, getting some exercise? How about doing an evening class at your local sports center after DH is home so that you can have adult company and exercise. Or if that's too pricey, then see if anyone locally (mumfriends, through MN or FB, etc) wants to do Couch to 5k with you, or even just start with a walk yourself.
In terms of tiredness, make sure DH is doing his fair share with DD - its easy if you are at home in the day to end up being the default parent in the evenings and weekends too, but this should be shared between you.
In terms of money - besides looking at the work situation - have you and DH sat down and looked at all your income and outgoings, made sure there aren't savings to be made (utility bills, unused subscriptions etc) and made sure that you both have equal disposable income? Would you consider doing something freelance/self-employed from home in the evenings? Something like selling books/beauty products/er... adult toys 
etc through parties could give you a bit of cash and some grown-up company.
My main point is to concentrate on your own life not your SILs - otherwise you'll just get bitter and begin comparing your DDs (as you have already done with sleep) which is unfair on them. I suspect that she has not/will not always have a kid who sleeps through the night but even if she does she might find other aspects of parenting tricky - toilet training, tantrums etc.
I really doubt her life is perfect, I imagine that maintaining the great job, partying, slim figure, perfect mother in the eyes of her parents act etc takes a lot of work and stress to achieve.
As for money, anytime I'm in danger of being jealous of the way others spend their money, I remind myself that I don't see their credit card statements or what they are missing out on to achieve it.
Concentrate on creating a life that makes you happy, I doubt its identical to your SIL.