Two people shit into the same toilet bowl
When the toilet was blocked, Kirsty and Alan had no choice but to become Shit Sisters
every family has one....the less favourite or popular or successful sibling. Think of Danni Minogue compared to Kylie. Danni is the shit sister. Could also, be the second favourite son, if you have a brother.
Danni is the shit sister to Kylie.
I have a shit sister, means to have a sister that's like a stranger to you.
Me: I have a Shit sister
Friend: oh damn want a new one??