I think this may be the one single example of a time when parentification would be justified. She wants a sibling? She can raise it herself.
Also, every single person I have ever known my entire life fought like cats and dogs with their siblings growing up, ranging from cruel jokes and name calling to straight up broken bones. Why would you want to deal with that on purpose?
It also sucks for the older sibling because they constantly get told to "set a good example for your brother/sister." They have to share everything with their sibling as well, including having almost all attention diverted from the older child to the younger child. I'm the only only child I know out of everyone I know.
If the parents knew they didn't want more kids, they should have sought more permanent contraception instead of relying on condoms anyway. Condoms do have a failure rate, even if your conniving cunt daughter isn't poking holes in them. And yes, if they didn't want another kid, they should have terminated. But then if they weren't wise to the daughter's shenanigans, how many more times would the mother have gotten up the duff with an unwanted pregnancy before taking further measures to prevent it?