u/Nyseme_Ptem

When Sivi has her conversation with Szeth at a bar in Mokdown, she steals a cup. There's only one other character in the Cosmere who likes cups. AND Sivi is the only Honorbearer to die naturally instead of being killed by Nightblood, meaning her soul was not immediately consumed. Presumably, her soul survived death (perhaps becoming a cognitive shadow or by some other method) and traveled to Lumar. This was obviously a long and arduous journey, and so her soul was greatly depleted, and only a tiny slice of investiture was left when she reached Lumar. That tiny bit of investiture attached itself to Tress' pregant mom and became part of Tress' Innate Investiture, and that tiny sliver? Sivi's compulsive attachment to cups, transferred to the unborn Tress!