You're very welcome. Wow, I am so sorry to hear this - it sounds very upsetting. If you believe your nephew is in danger, it is imperative that you get him in a safe place. What I mean by this is contacting Child Protective Services or informing your brother-in-law of your concerns. I cannot say she's going to harm him or not, as I haven't met her or evaluated her; yet, your concern, concerns me. Hence, why I've suggested you speak with someone in your area about your concerns. If you have any evidence (text messages, written letters, facebook posts, etc.) that can help back up your claims that would be helpful for them as well. Is your sister and BIL doing a custody evaluation? If not, I would suggest this to your BIL, as they'll both get forensic psychological evaluations to determine parental fitness. This can help with your nephew going to his father vs. mother. Again, document EVERYTHING - this can be helpful in court.
I would call them, let them know her car/make/model/license plate - the next time she drives around with alcohol. This will help in the court proceedings because she is endangering a child, and safety of others - this will also help with evidence (police report) to support her being an unfit mother.
That's definitely possible, again, you want facts - so if you're ever in her home or your BIL is - get evidence to support your claims. You can also call the police to do a welfare check and say you believe she is using - they will go and assess the situation.