Nintendo isn't traditionally known to develop worlds with very sad or dark histories, but Animal Crossing: New Horizons is a slight exception this rule. This is due to the fact that two of its mainstay characters have fairly sad family histories.
The characters in question are Sable and Mabel. While the two aren't generally the most cheery or whimsical characters, they aren't exactly what most Animal Crossing players would think of as having sad backstories. This couldn't be further from the truth, as it turns out that the two sisters were two of three sisters, whose parents died when Sable and Mabel were both young.
This all is revealed through a discussion had between the villager and Sable upon approaching Sable in her shop. It seems as though her explanation is unprovoked, though she nonetheless explains that the pictures on the back wall are of Sable and her sisters. As well, there are pictures of Sable and Mabel's parents. However, Sabel reveals through dialogue that their parents died when they were young.
Mabel then chimes in and adds that, despite this, Sable did an effective job of helping to raise Mabel. Sable makes light of this, but the moment mostly contrasts Animal Crossing: New Horizons' whimsical and happy tone. Of course, players will encounter the characters with sour attitudes here and there, but this is fairly unexpected of Mabel and Sable.
There are other Animal Crossing characters with backstories that are either resonating or evoke real-world problems. For instance, dialogue in past Animal Crossing titles will reveal that Tom Nook may not be the corrupt capitalist many take him as. Rather, Nook himself had been prone to scams and was refused a loan due to his being a raccoon, evoking the concept of discrimination.
However, it is still unexpected for Nintendo to put a backstory of tragedy in Animal Crossing. Even so, it still tends to get dealt with in a way that doesn't downplay the thought without depressing the player altogether.
Animal Crossing: New Horizons is currently available on Nintendo Switch.