This is very tricky, because naturally your family want to celebrate the arrival of a new baby and make a fuss of your sister because it is hard work being a new mum. On joyful occasions it can be very hard to acknowledge someone else's pain though, and people often don't know what to so or say for the best. So if, as you state, you appear happy and content and 'over it', perhaps fhe family is taking their cue from your brave front. It is an easy way out, but people might not want to poke at what is obviously a painful situation, and risk upsetting your carefully constructed composure.
Unless they are typically very heartless people, I doubt they have forgotten everything you went through.
What was your relationship like with your sister before she has the baby? Do you actually want to be around the baby and get to know the baby, or is that a step too far? I'd consider talking to Dsis a bit about how you feel, just say you've been finding it harder than you expected to see her with your niece/nephew, and it has reminded you of what you missed out on when your DH was sick. She might prove very understanding if you can find some words to start the conversation, and then you can decide together if it would help you to step back from some family occasions for a while.
It sounds like your big problem may be your mum, though. What is it you would ideally like her to say or do differently? Whatever it is, you might have to start dealing with the reality that she isn't going to be the mother you need her to be at this point in your life. Has she always been a bit rubbish when you've had problems? Did you feel like you and your sister were compared a lot growing up?
Many mums seem to go through a phase of jabbering on about how much they want to be a grandma, or endlessly talking about their friend so-and-so who has just had a fourth grandchild. If grandbabies don't come tumbling out of their own children, there is disappointment, sometimes it is palpable and stated directly, sometimes it is more subtle. Your mum, having now realised her great hope now rests with your sister, may have transferred all her energy there and that could feel very hurtful.
My best advice here if you can't get anywhere with DM or DSis, stop trying and withdraw from these family occasions for a while. You don't need to put yourself through it. And if you do feel like you want to attend then stick to family members who will chat to you about grown-up things and not talk endlessly about the baby. And don't feel you have to dandle the baby on your knee and coo over her or offer babysitting, if you don't want to. Excuse yourself to make a cup of tea or go and look at the garden or simply get your phone out and distract yourself. You don't have to play happy families, if it is making you feel bad.
In a few years when the baby has grown up things will likely simmer down. Certainly my the time my DD turned 7 she became just another member of the family to chat to. If you don't want to be close to the child, you don't have to be.