Etymology aaaaalmost helps with this one.
Proto-Indo-European * swéḱuros is one's father-in-law (giving Latin socer, Italian suocero, Spanish suegro). But that became Proto-Germanic * swēgraz "brother-in-law" (giving Norwegian sviger via Old German).
I just have to remember that it's off by one generation.
I've encountered an odd thing. On a listening exercise, I wrote Svigerinnen min er snill. as I was not really expecting to hear the feminine form and didn't realize until after I submitted it. The error chime sounded, the message was in red, but the text of the message was the "Nice! Meaning: My sister-in-law is kind." as if I had been correct.