My 9 year old nephew asked me to buy him wine cooler and I didn't tell his mom.

Sorry for the weird title format, app is being stupid.<br><br>My (17m) nephew (9m) and I (17m) are very close. When my nephew visits he usually only wants to play board games with me. My sister is a very strict mom. I don't always agree with how she parents him but It's not my place to question it. One night she had left to go grocery shopping and my nephew asked me to buy him a wine cooler. I asked why and he told me he wanted to feel like a grown up and drink a wine cooler like the adults do. Without telling my sister, I took him to the liquor store and bought him one. I told him not to tell his mom as she would kill me if she found out. We both finished our coolers and kept it a secret. The next time my nephew visited I bought him another one. It has now become a tradition whenever he visits and him and I will have wine coolers and pretend we're grown men doing important business. I don't think it's wrong as it's just a grape or berry flavoured drink but I don't want my sister to kill me.