Hun I know exactly how you feel. I have four older brothers and none of them have got used to the fact that I'm pregnant let alone that I'm keeping it. It's really horrible coz I all I want is them to be able to share it with me and actually maybe just talk to me about it and get excited but it's not happened so far.
As someone said earlier, your sister probably just needs to get her head around it and get used to the idea. I know that the problem with my brothers is that they feel very protective of me, maybe thats how your sister is feeling as well?
I'm still hoping my brothers will be better about it, and I'm sure your sister will as well!
Hope things get better for you hunni

xx