pretty lame frankly.... more like bullshit to smoke her out. After sometime, boredom kicks in, instinctively, we like variety.
As sm mentioned, technically, it could be possible. In that case, its just psychological mostly. The more likely reason is, he is just finding a lousy reason to break up after he is 'done' with her. He doesn't want her brother coming after him, so, he needs a reason to make her let go by herself.