There are those unwritten rules about who we should and shouldn't date around friendship circles. There are girl/bro code rules that give guidelines on boundaries we are supposed to observe in the dating field.
For instance, the bro code dictates that a bro should never date or hit on a bro's sister or mother. I'm not sure what the sister code says about this, because I've come across 'sisters' who date and boldly shag sisters' brothers.