This means Leah and Lilith are cousins.

In The Sin War books, the original Nephalem became embroiled in a plot between the angel Inarius and his lover, the demon Lilith.The two, along with … Lilith in the Diablo series of video games, is the daughter of Mephisto, sister of Lucion, lover of Inarius, mother of Linarian; and known as the Queen of the Succubi. Diablo 4 is the next big hack-and-slash RPG from Blizzard and while we don't yet have a release window for when it'll be available, there's a lot to read into so far.

However, she cannot be considered as the Demon Queen because Andariel already holds that as her secondary title. Now Diablo and Mephisto are brothers.

Lilith is the mother of all humans in the Diablo vese though so Lilith is also Leah Y times great grandmother. The Nephalem. As the daughter of Mephisto, the Lord of Hatred, and Queen of the Succubi, Lilith’s history within the Diablo series should not be understated. Leah is the daughter of a Diablo possessed Aidan. The identity of her mother remains a mystery. Lilith is mentioned four times in the Babylonian Talmud, but it is not until the Alphabet of Ben Sira (c. 800s to 900s) that the character of Lilith is associated with the first version of Creation.

So Leah is kind of Diablo lord of Terror's daughter.

In rabbinic literature Lilith is variously depicted as the mother of Adam’s demonic offspring following his separation from Eve or as his first wife. With the formal announcement of Diablo 4 finally out of the way, it appears that the leaks involving Lilith as a key player were also correct. In this medieval text, Ben Sira names Lilith as Adam’s first wife and presents a full account of her story.