I think you have to try and look at the positives... You will have the first grandchild so that is very special. Your little one will have a cousin close in age, so when you see your brother & family, they will have each other to play with.
I got engaged before my brother a couple of mths before they did. We married almost a year before they did. They had the first grandchild in the family. We were 3mths apart in our 1st pregnancies. 6 weeks apart in our 2nd pregnancies, my brother having the 2nd grandchild first. Then I fell preg last year but unfortuntely lost our baby at 19wks in Jan 08. At the time I lost our baby, my Sil was just 12wks pregnant with their 3rd. I didn't know, she told us a week after we lost Isacc... They now have a beautiful baby girl...
Its probably pregnancy hormones making you feel a little the way you do. Give yourself some time to get use to the idea. Enjoy your pregnancy and concentrate on it. Your Sil will probably come to you for advice & someone to talk to as her pregnancy progresses and even ask you about the birth. The main thing is at the end of it all, you will have a beautiful baby of your own and also a beautiful niece or newphew and same goes for your brother.
I must admitt I was extremely jellous when I found out my Sil was preg after we lost Isacc, but in all fairness she has suffered m/c's herself and it was better to be happy for them then to be bitter and upset. It was only a waist of my energy and time and didn't make me feel like a good person. I know she felt bad & didn't know weither to share the pregnancy or not. I made a concious effort to make her tell me how everythings was going. I mean this was my little niece my own flesh and blood.
I hope keep enjoying your pregnancy, soon you will be a mum!
Wishing you all the best