first of all, don’t do it if you’re not legal. if you are legal, think about your comfort level. are you okay with posing topless in front of someone that is going to paint you? are you comfortable with this friend? some people are more comfortable about posing topless than others. i had something like this happen a few months ago where my friend asked me to model topless for his photography project and i was hesitant but i did it because i was flattered that he asked me to do it. it’s really up to you. your family doesn’t necessarily have to see it, and if you’re legal it shouldn’t really be a problem it’s not like you’re doing anything inappropriate.
You can actually photograph underage people naked if it is art, painting would without a doubt be legal unless its of sex.