I have a feeling if you tell your parents first... your sister will find out at the same time if she still lives at home. Be open and honest with all of them. That is the best way. Talk with them about your options and expectations. Good luck and congratulations.
I would tell your boyfriend first then tell your family with your boyfriend by your side and your sister will find out by your parents or being around when you tell your family you are congrats and good luck =)
to BF : I am glad to inform you that the RESULT of our love has come as my gyanecologist has confirmed that I am going to be a mother of your child. Parents : Send them a beautiful doll and tell them you got promotion and from parents you have become grand parents of a beautiful baby doll. Sister : Send her the MMS of your ultrasonography and inform her of a new doll coming in the house. BUT DON'T IGNORE YOUR INLAWS.