the thing you have to understand about laudanda is that she's 81, as both the eldest sister and being considered a nice respectable early Real Adult by merfolk terms, and has, for the majority of her life, been the Crown Princess.
this informs basically everything else about her — unlike her sisters, she was the first-born and grew up without any siblings. she was always conditioned as the heir to the throne and brought up exclusively within that context, being extremely isolated from other merfolk her age due to political positioning and the movements of the whims of the courts. when she was growing up, the land was starting to be discussed as an actual entity that the merkingdom had some interest in, but it was treated the same as other political maneuvers and topics within the merkingdom, not as something that applied to laudanda and not something of interest to her beyond what she could gain from it. she was raised in the most standard way for any merfolk of her title, by a team of servants and nannies who oversaw her health and her care and reported directly back to the royal family to ensure she was being cultivated for her title in the best way possible.
inheritance for low royals goes by youngest, but it's not a guarantee that any heir has siblings. most don't, or have a low number of siblings, something which is considered more ideal, because then you need to find somewhere to put those extra kids where they won't cause problems, and because it does, subtly, imply that your attempts to cultivate a heir repeatedly failed. the current line having a set of four siblings is considered highly unusual and highly suspicious for this reason.
so when laudanda began to falter in her studies and started to appear much less ideal as an eventual claimer of the throne, amanda was planned and born as a replacement.
this was... a bad time. a bad time, because all that raising and training and priming to be the crown princess meant that this was the end of the world for laudanda. this was everything she had been born and raised for, everything she was supposed to be, everything she had been told constantly that she had to be, and it was being taken away from her, taken away because she wasn't good enough at being it and had failed to be everything the world had primed her for, and taken away by a little squirming green thing that didn't even have the good graciousness not to somehow fuck up and be born with an obvious color mutation.
laudanda, who had been pushed and conditioned to view her only worth in the entire world, the only reason she existed and the only definition of herself that she had ever known, who had been pushed constantly to defend it and protect it with everything in her body, with life and with limb, that any manner of cruelty was justified so long as it was done in the name of the title, in the name of seeking that perfection, only saw one way out, and it was taking it out on her younger sister.
although having multiple children can be seen as a failure to produce proper heirs, encouraging those children to fight for that title and put pressure on each other to force their siblings to renounce that title, IS seen as a good manner of raising heirs. effectively, it's a smaller version of the political jostling and fighting that the adults in established titles do, and being able to accomplish that at a much younger age makes someone a lot more appealing as a potential heir. they already proved they were willing to do anything to remain loyal to that title, and already proved a skill in positioning and convincing someone else to give them what they want. siblings aren't common, but sibling infighting regularly is, and royals not-so-subtly tend to pit their children against each other, as a means to prime and encourage them towards other forms of politicking.
it just also happens to turn out that this sibling infighting tends to start very young, and often while the older sibling has multiple decades on them in age.
but, hey, laudanda got what she wanted! she got amanda to renounce her claim to the throne and to swear instead to a religious title that would inherently exclude her from the line of inheritance! isn't this what everyone wanted, in the end? laudanda secured back into her title, amanda blissfully exempt, neither having to worry about the race to that finish line?
well, until bellanda came along. but that's a story for a different time.
