Now there's an additional motivation for someone tampering with the will: it could be that someone (probably not Oji-san since he brought it up) wants Naomi to inherit the company as the "male heir". Japan still be a country that leans toward sons. It could even be someone wanting to protect Yuzu as she'd have to be married off to secure a strong partner to take over the company if Oji-san wasn't up to it or wasn't old enough in time.
Naoomi is also a better choice as a direct male heir of the late president of the family's company/corporation. Mikiomi is the dead dad's sibling, and even if not a child, a child of the most-recent president would be ideal for the sake of succession.
If Naoomi hadn't shown up and Suzu wasn't married off to someone brought in to "take over", Mikiomi would have been the next in line; and as a child, he'd likely be subjected to some stewardship (maybe from Awashima Tougo), like how children ascending to the Crown in a monarchy would have a regent oversee things until they came of age, type thing.
But that will was fake from the get-go; which means that whomever found Suzu's fake will and amended it is playing a larger game. The question becomes how deep does the plan go--if it's all because Suzu stumbled upon Naoomi as a kid and Unknown took advantage of that fake will and is flying by the seat of their pants through the current narrative, or if Naoomi's existence and the dad's double family was known before even that, and this was all part of some even more convoluted plot involving warring factions for control of the company and whatnot.
It's honestly more intriguing than the question of whether Suzu and Naoomi are actually related. Though if this
is some super deep 5D Chess game of succession, there's a good chance that Yukari and Suzu's parentage play a part in that.