How to respond to questions about not drinking?

Ann Marie • 35, Boston area. I have a 2-year-old son, pregnant with my second, due July 2018
I've been TTC for a month, and my brother's girlfriend asked me why I wasn't drinking when we met them for dinner. I'm in my TWW, so I told a white lie that I had a drink earlier, but she implied that I would only be not drinking if I was pregnant. I think she was rude to even ask because, honestly, am I going to announce that we're TTC right at dinner? Do you think it's rude to ask? She and I are not that close, and my brother doesn't want to hear about TTC. Also, although she has no kids, she's in her 30s and should know better.