How Do I Get My Son Off Drugs?

My brother was going to marry this girl last summer. After 1.5 years of great relationship, blah blah blah. 2 weeks before the wedding, it turns out she is a crazy closet drug addict.
She has a little girl, the daughter was my niece for 1.5 years, she is 2 now. I have known this baby almost her whole life.
Anyway, in August the relationship broke up. She was violent and abusing drugs around the kids (my brother was a single dad, my nephew is 3) etc. My brother notified CPS, told them of her drug use and violence. We heard nothing from her for 4 months.
Now, tonight, I get a call. She finally blew it completely. She took the baby out with her, partying…the child has been in foster care for 4 days. She wants me to take the baby while she gets her act together.
This is where my problem is. I have done this before. I raised someone else’s baby for 10 months. From 5 months to 15 months. Then the parents got ‘better’, I had to give the baby back. This nearly killed me.
I would love to take this baby on. My niece. But, her mother does not like me (the feeling is mutual), and I know if (when…) she gets her daughter back, I will not be given access.
I want to help. I hate that she is amongst strangers without anything she knows, but I don’t want to have her, get attached to her, and lose her.

Any ideas, tips, suggestions? What should I do??? I feel like I am being selfish by protecting myself…

My husband is more than willing to take her in, so that is not an issue, and I have 3 other kids.
hwnp808—I really hope that happens, but the deal in Canada is they (the parents) get a chance…3, 6, or 12 months. It only takes hours to get attached…

Also, I should mention. When I said “She wants me to…” I meant the social worker…not the mother, I haven’t heard from her. The Ministry contacted my brother, and he said I am approved for foster care and safe home. So they called me.
Such good answers.

Heyy the Shortay-I know. No offence taken. I am only having trouble with this because I have done this before. It really hurts. I don’t know all the details yet. I have only spoken to the social worker once. I have a phone meeting tomorrow.

I have been “married” for 16 years, I never loved this man, I hardly had sex with him after signing the “marriage license”, I was pregnant when we signed it.
After leaving him and comming back trying to make the situation worked for several ocasions, the only affected has been my teenager son.
Now, that I had spent all my savings trying to save, house, cars, bills, and even him from jail for having 3 dui’s and suspended Drivers license, he is treating us worst and worst and worst. I have no means to get divorce, since I am unemployed ( he is too)
We hate each other and I notice that he does not love my son, he never never never never talk to him, just mumbres Hi and bye.
He is his son, but neighbors and classmates think that my son has a step-father because the way he treats him: he ignores his son completely, unless he wants some of what my son is eating, my husband even dares to ask for it).
please help me
I am desperate, he stole my life because of my weakeness, my compassion towards him, my stupidity, my fears,
please let me know if there is a posibility
thank you