And in the beginning I thought BiL did really well. He didn't challange the metaldragon, but identified it as a traveldevice, he was cautious of grandpa (all he knew he totally could have been a danger), but didn't turn that meeting into a hack'n'slash-fest. Overall cautious, but also openminded. He did really well, until...
I get why he was hostile ("My little baby-sis has to be protected from this man."), but why was he hostile BEFORE he knew about the marriage? Directly afterward, after Chris splashes that fact into his face, okay. But before all he saw was a happy and health surviving Chris, being well so far, and a man, she felt relexed around. First thing is of course treatening and insulting him. I see why mage girl isn't in a relationship with him, eventhough they have traveled the other world together. Stay away. That's a red flag to avoid.
Though going in, snagging the price and going out is a praiseworthy strategy. Maximaizing the gain of the mission, while minimizing all possible threats. Yet he still should listen to his sis.