The similarities with Rosetta would be the documentary touch, and the closeness to reality. But while the Dardenne's kind of made an allegoricall film about the "state of the world", In between days is much more personal. It also totally differs from the urging frenziness of Rosetta, with a very calm and deliberate pace (I can't imagine people being bored or falling asleep during Rosetta ) Both characters can't express themselves or communicate properly with the world but Rosetta is much more extroverted, than the introverted almost invisible protagonist of In between days. The ending of both films is very similar, though it points again in two different directions. While for Rosetta, the world might have opened up a new perspective in a cathartic "finale" of her emotional tour-de-force, In between days offers a a world-weary truth closer to life. Sometimes just nobody cares about yourself, and you are again at the beginning, as you are the only person who can truly help yourself.
I prefer the toned-down In between Days, though the direction of the Dardenne brothers in Rosetta is in a league of its own.