Alexei had never completely known 'safe' - even here at Hogwarts. It was always in his mind that the war was not over. That Lady Nyx would rise again, and he was to serve her when she did. He was of age now, even if he had two years left at Hogwarts.
Alone would have been better - entirely alone not this. His room would've been better. "Yes. Family and all." It was better than the alternatives though. At least being near Sera meant being near family.