It's his sister, and I think Alina told him she wanted to pack, so he's staying out of the way.
They aren't splitting up or anything serious, they are all still laughing together and being too nice to be doing that.
The person I referred to definitely said "Alina's sister" and I can tell you right now, the fat chick is not Alina's sister. Now...if you are claiming the fat chick is Anton's sister, I suppose that could be...though I haven't heard that being considered or proven.
It's definitely been discussed here, not sure about proven though. She has the same smirk/smile as Anton, and I see a clear resemblance. My opinion anyway, for what little worth that has.