If you watch the series, it seems Tetsuya is so obsessed with what he thinks is his only reason to exist -fighting the enemy- he is completely oblivious to Jun's obvious advances. But it is UNTRUE! In reality they began dating long ago, but since they believe their father and friends would be disturbed by the idea (despite they are not siblings by blood), they are keeping it from everyone. It is very unfortunate for poor Boss, who does not know he missed his chance with Jun long before meeting her.
Often Venus A gets defeated easily for the Monster of the Week. Still, Jun took down one of the generals of the Mykene army with one single Eye Beam! And in the Nagai Heroes e-manga (a gigantic Crossover featuring Go Nagai series), she kicked Tetsuya's butt and nearly gutted Great Mazinger open as brainwashed. So she must pretend she is less capable than she is. Why would she do that? Because she knows how fragile really is Tetsuya's pride and confidence, and she wish not hurting him. And she also knows how annoying and ornery he gets when someone shows him up, and she would rather saving herself from the headache.