Yeah, I was willing to see Obi as potentially keeping his love a secret and serving both Zen and Shirayuki faithfully forever, but she totally knows, and Obi seems like he's getting less and less platonic too. With this manga I could almost see Zen being okay with the situation, but realistically, this isn't going to keep working. At some point Shirayuki's going to say,
"Hi Zen! Guess what? I'm getting married to Obi next month, and I decided I'm not serving as court apothecary anymore. So, good luck getting your brother to let you marry some other girl you decide you love in the next 10 years. Hey who knows! Maybe he'll let you see the next one more often than once every 6 months."