Being a writer is a solitary profession. You don't get in a car and drive to an office and sit at a desk to get paid all day. You probably never have to leave your house, unless you are doing research. There is so much that can be done online and over the internet now.
So if your family and friends know you never leave the house to go to 'work', then the question becomes, should you tell them what you do? Do you tell them you are a writer?
While sharing this information is certainly helpful when marketing your books, or networking for freelance work, it opens you up to risk of criticism. Not everyone is supportive of your career. Some may laugh at your attempts to be published. Others may continually interrupt you because they don't consider writing a serious profession.
While nothing is predictable, you know your family best. How do you THINK they will react? If you think they will be supportive, then by all means, tell them early on. If not, then hold off until it's absolutely necessary to tell them.
Whichever you decide, be prepared for the worst case scenario. That way you won't be disappointed if they react unfavorably, and you will be surprised when they are accepting.