AITA for possibly taking it too easy on my little brother after he "spiked" our family friend's drink?

I'm a 22m college student right now, and my little brother is in grade 9. For the past 5 days, our parents have been overseas, so my bro and I are staying at home, and just because I'm studying for the MCAT and really stressing out, I'm being helped in looking after my brother by our long time female family friend named Rina, who turned 44 this summer. I myself have known Rina since I was 5 years old, and I actually see her sexually. So does my little brother. Despite this, she seems to be totally unaware of our feelings towards her, and she openly tells us to consider her our "older sister". We like it because she's a very kind person, and she's really, really nice to both of us and to our family. She also always really casual with exposing her skin, and she doesn't seem to know the effect it has on my little brother lol.

Two nights ago, Rina invited her boyfriend over to our house, with our approval and my parents did give her permission to do this if she felt like it. I'm not sure if she was allowed to do this, but she helped herself to our alcohol, and got quite drunk while her boyfriend was the designated sober person. Whenever Rina gets drunk, she just gets utterly adorable, and I think it really pushed my brother to the edge. Because last night, in an effort to see her all cute and letting her guard down again, he spiked her drink with alcohol. She saw right through it, and it really freaked her out, even though it didn't do much harm to her (she stopped drinking immediately). She got my brother to fess up, and she didn't take it very well at all.

Today, I was at home, studying, when Rina walked in. I could tell she was trying to put up a brave face, and she told me that she was waiting for my brother to come home so that she could chat with him. When he did come home, however, she kinda froze up. She tried acting normal, and tried making us dinner, but she started freaking out and puked a lot. Her cooking is always amazing, and this was the only time my brother and I weren't comfortable with eating it....I just now walked in on my brother lying in his bed, crying, and I did my best to comfort him, and patted him on the head and managed to get him to calm down....

Am I doing this right? I have more than a week left until my parents come home.