In 1938, the year of this film, one Swedish kronor equaled about $0.25 U.S. So, Louis' gambling debt of 50,000 kronor equaled about $12,500. That amount, in turn, equates to roughly $243,000 in 2021.
The play, a comedy in three acts, was published in 1926.
Even though the premise of the film is that the three couples are all related to each other by marriage as brothers- and sisters-in-law, it's never actually laid out who is related to whom as siblings. None of the men share a last name, but could conceivably be half-brothers. Sussi specifically refers to Julia as sister-in-law (not sister), and her husband specifically as Julia's brother-in-law, suggesting that Sussi and Kurt are sister and brother. Julia and Ludvig are pretty much the only man/woman pair that doesn't flirt with one another, suggesting they could be sister and brother, but it could also be that Julia and Katja are meant to be sisters given their similar appearances (but different body types). It's also possible that references to each other as "relatives" is more of an honorific since they had all been friends so long, but that seems unlikely in Swedish culture.