Young Mum dumping her 3 year old off every wkend to party with drugs. NEED ADVICE pls just hear me out!?

Well, my sister who is 20 has a 3 year old boy. She works a full time job, and he goes to childcare, So everyday she'll only see him for 4 hours of the day before retiring at night. And will only see him for a few hours on sunday when she picks him up from my place the night after shed gone out. Im 22 and have 2 boys myself, BUT she relies on me too much to pick her boy up from child care and take him for the night while she goes out, sleeps with guys, takes drugs and drinks up. (and having 2 boys myself is a tough enough job as it is let alone having a 3rd every 2nd wken) It started off with just me havin him here and there, after the father of her boy left them for bashing her up. So i felt sry for her n told her just to go out and meet someone nice. So since then, shes takin advantage of the fact that im fit enough to take him ALL the time and expects me to do it with the click of her fingers. Yesterday, she took speed while at wrk and called me to ask if i could pick up her son coz she mentioned that she was tpp paranoid to pick him up. I stated that she should have thought of that before she even touched it. All she could say back was "i didnt think". how can you not think and forget that you had to pick up your own child from daycare? And just before she told me she wld give me 50 bux to watch him tonight, right after i was saying i have a horrid migraine and wanna put my 2 boys to bed and just go to bed myself. She gets so desperate that she'll get anyone or pay anybody to watch her son just to sleep and be with guys and drink n take drugs every wkend... Im really lost with what i should say to her, how should i approach her and explain to her what she is doin is WRONG! and that her son is totally traumatised by what she is doing.