Hiya Rach
I'm going to be really honest when answering this but I am aware that sometimes i don't phrase things in quite the right way so don't be too offended!
I don't necessarilly think you are being selfish but I do think you should stop feeling second best to your sister, because you aren't second best! Families are really difficult things to live with, mainly coz you can't choose them but don't waste your life worrying about what other people think of you and what they do for you - you are just using up vital energy you could be conserving for you and little one.
Ignore your sister and just let her get on with her life and you get on with yours, she probably doesn't even realise you feel overshadowed by her. She can only take away these special times for you if you let her.
Be happy and excited by the new life you are embarking on with your baby and soon-to-be husband because when you focus on that, you won't give a stuff what she's up to and who's helping you out organising the weddng etc.
Families will never match up to the expectations we have of them, and years of experience tells me that when I stopped trying so hard to get attention, help, support and praise from my family and only relied on myself to make me happier - that's what happened!!
Being as pregnant as you are also won't be helping and I can sympathise there as I am 36 weeks so also feeling uncomfy, irritable and emotional so I suspect that will be playing a big part in the way you are feeling!
You've got so much to look forward to, try not to feel down about everything.
Sorry if this sounds a bit like a lecture, it's just having spent years going through the same thing I can see it from having come out the other side too!
I hope things pick up for you soon
Take care
Tracey