I need advice on telling my boss!

Hello everyone, I am currently on holiday from my work (I go back next week) I need to tell my boss I'm pregnant as I work in a section that involves a lot of heavy lifting. 
My problem is that I am not planning on telling anyone im pregnant until the 12 week scan, I also work with my sister and I know if I tell my boss he will tell someone and word will get out, I would hate my sister to find out through gossip in work, any ideas?