So, first, he is told NONE of that backstory lol. I think his friend/sister's husband tells him she disappeared, maybe with a man? To stop him from trying to find her, because she actually disappeared into the dungeon to look for him. He did this, not in front of MC's sister/his wife, btw, because he very purposefully lied (and gets the shit beaten out of him by MC later lol for it) because he didn't want her to have that same grief the sister had when he didn't come back. MC and sister only had each other after their parents died, so she was absolutely devastated when, for all they knew, he sacrificed himself, and he didn't want her to have to go through that again.
There's still the question of Haruka, who I don't believe they told MC about her and Rinka's connection. It's very up in the air about who or what she is, but, and I don't remember if this was said in the manga, Rinka was the great sage that brough magic to the world's granddaughter, so it's implied some magical shenanigans. When the true story is revealed, either through a viewpoint chapter from non-MC's POV, or from telling him the truth after he gets hints that she's in the dungeon, I think I remember them talking about how her just shacking up with another dude was super OOC for her, so they're still not sure wtf is happening, but Haruka's existence was kind of the beginning of MC's sister getting out of her slump.
Also, the goblin swarm thing is WAY better done, because there feels like actual stakes, and I don't remember the magic gun thing, either.