There isn't a foolproof answer to this question... Personally, i have never tried to convince anybody to invest in anything. Not my parents, not my brother, not my friends...
If it goes wrong, they will always feel like you were responsible for their loss, and that's a burden i don't want... But that's just my personal opinion.
You're right mate. This remind me of an incident that happened some years back. A son advised him mom to invest in shares and the mom took the advice of the son and bought some shares. After some years, the company hit rock bottom and it affected the shares of the company. The mom has been disturbing the son and blames him for the financial misfortune. Because of this, sometimes I find it difficult to advise a family member or friend to invest in a business or project.