Boring jokes about women dictators aside, we really have no idea who’d take over if KJU died. Being a woman is actually a pretty big demerit against his sister in a governing culture which is very militaristic (and doesn’t have much of an independent power base anyway...) and his brother doesn’t seem to give a crap about governance.
His sister doesn’t have much of an independent power base and it’s more likely that you’d get a major conflict within the party over who the successor is, and the consequences of that could get pretty ugly for the world at-large. Definitely easy to imagine that we get back to the climate of 2017 where we see a lot of posturing by NK in some general’s effort to consolidate his control and worst-case, now would be a terrible time for a regime collapse scenario considering the pandemic and deteriorating US-China relations.
As weird as it is to use monarchical terms to describe an allegedly Communist government, I could see Kim's sister leading a "regency" for Kim's 9/10/whatever year old son until he comes of age. The son would technically be the head of state and would eventually mature into power but she'd hold the reins until he comes to adulthood.
