Congratulations!! You are so sweet thinking about your sister like that. But this is YOUR exciting news, she will get used to the idea and should be happy for you!!!!
You're just lucky you are super fertile
You should tell everyone soon, maybe at 12wks? And expect people to congratulate you and be happy for you as this is nothing but happy news! If others can't be happy for you or they are jealous than that is there problem hun