I would bring rum and screw HER. lol

 
Yes, technically you are "contributing, blah blah blah" but personally I take some laws a lot more seriously than others. Anti prostitution laws being pretty much at the top of that list, right along with worrying about the "illegal" drinking of a 20 year old.

 
I am sure I don't need to point this out, but I guess it bears mentioning that you need to be sure that she won't be driving after the session, the last thing you need is for her to get pulled over/arrested/accident and for her to claim that you supplied the alcohol that got her drunk.

So I lean towards no booze.  
But I do love to tipple a bit, so if they insist, I give in.

I let them drink or do whatever drugs they want. Inebriation is a great excuse to drive them around for a bit afterwards and bang them again. Had one SB who had a half glass of wine and claim to be too tipsy to drive even two hours after so she cancelled her plans to stay with me. She obviously just wanted to fuck more, so the alcohol was a great excuse. Ended up canceling a 2nd SB so I could bang the first one overnight. Saved me $300.  

Obviously, don’t let them run around or drive drunk. I’ve dumped sugar babies because they keep showing up with too much alcohol or drugs and get out of control.

But in the world of broken toys they all seem to have a love of 42o, alcohol and what else. If they are fun its great. But many fet angry or sad. Then its bye bye.

I wouldn't get involved in any drugs/alcohol early on.  And no drugs later.  But for someone I've known for a while, some underage alcohol would be okay.  As the supposed adult in the room, I'd monitor it though.  If bad stuff happens your name will be in all the papers -- and not in a good way.

with a really sweet SB who insisted on drinking wine during our sessions.  She was of legal age, for drinking as well as for sex, so I was ok with it.  She said she needed the wine to relax before sex.  Maybe that should have been a warning sign.  Anyway, we did this many times and she always drove herself home fine.  Except the time she didn't and wound up flipping her car and hurting her arm. I felt terrible, since I'd given her the wine and had not realized she was impaired when she left me.  Since then, she has changed her ways somewhat.  She still want the wine, and we still drink some during our sessions,  but now I pick her up and drop her off.  She's serious about not drinking and driving anymore.  Which is good I suppose.  But since that incident, I don't offer alcohol to any of my SBs.

You can bring rum for yourself and screw her but don't offer her any booze during the first meet. The next thing you know she is threatening to call cops on you for offering her booze so you could take advantage of her, I have met a couple of BSC SBs who try to act up in BCD meetings especially the under 20 ones.  
Of course you can let her drink rum after you have fucked her and if she is spending the night with you but don't start with giving her booze if you are meeting her for the first time just as a precaution.

Thank you everyone for your advice and recommendations.  This SB uses Uber/Lyft, so driving under the influence is taken off the table.  I think I will screw her first and then have drinks during recovery time for round #2.  Not into drugs or SB's who use drugs.