My sister has struggled with addiction for many years, she's lost her kids (they live with their dad's) and been in trouble with the police.
She got into a rehab and got kicked out for taking drugs in there so we have kind of gave up on her getting clean. She bounced around from homeless place to another bit eventually got herself a flat.
She has now been sent to prison and because her sentence is over a particular threshold she can't get housing benefit and so is looking at losing her flat.
She has asked me to chip in towards paying her rent, she is planning to ask me, her boyfriend, best friend and her daughter to all club together.
I can afford it, I have a child I am supporting through uni and two children at home so it's not like I won't miss it but I could afford it if I want to.
The reason I am tempted to say no is - it is asking a lot of 4 separate people, if any of them miss a month or change their mind, then the whole lot has been a waste. She can only keep the flat if everyone pays (or someone else picks up the slack). So I can see a situation where everyone feels sorry for her in month one and two, then Christmas comes up and her best friend who has four children can't afford it, so I then pick up that payment too, she has an argument with her boyfriend and he decides he's not paying and before I know it I either pay the lot or. She loses the flat anyway and it's all been a waste of money.
So YANBU - don't pay
YABU - this is your sister, she will be off drugs when she gets out and this is a real chance to change her life. You can afford it so pay.