Oh hunny that really sucks about your sister and your best friend. Tell them that there is no other option and you understand their right to have an opinion but that you would prefer that they be supportive of you in YOUR decision.
If they are decent people who love you, they will come around. Give them some time.

I feel incredibly lucky to have a family like I do and the friends that I have because my support network will be great. It does not mean it will be easy for me. I'm sh*t scared still. But I know I'll survive. and you will too
Do what you think is best for your baby and others will come along and help you if they truly care about you, you'll see.

And to answer your earlier question, 7 days gestation is the same as 3 weeks along. If you were only 7 days along, theoretically you wouldn't have even done the deed yet and wouldn't be pregnant yet so the test could never have been positive.

I hope you feel better soon, and I am here to talk any time.