Context, from what I know or gathered. Everyone in this story's present-day cast are some monster or other, with ridiculously long lifespans. And communication problems.
The younger rampaging vampire (Flan) is emotionally unstable and has self-control issues, which her older sister handled by locking her up in a basement for several hundred years. Which caused a bit of a rift between them. At some point the human witch (Marisa) broke into the mansion and happened to meet her and they became besties, mostly by virtue of being really, really difficult to kill. This was a very positive and stabilizing influence on Flan.
Being human, the witch (and the head maid, referenced in flashback) eventually die of old age. Note that the first couple pages show Flan appears to have no concept whatsoever of someone aging or dying from age. So the rest of the mansion collectively decide to sit on this particular timebomb, probably because they are busy trying to come to terms with their own loss/grief (the maid). After waaay too much time passes without this situation being resolved (looking from the state of that cottage), Flan comes to the conclusion the witch stopped wanting want to visit her anymore. She decides it's time to go see her instead of waiting for a visit, come hell or high water.
The older sister has foresight and knows it's going to happen, so uses this to first establish a new hard-to-kill playmate in the guard, and then close the gap between each other by being there for Flan like the other residents were there for her when she was having trouble coping.