@Rouenpucelle Hmm, that's weird. I have a story in one of my saves very similar to your own: older brother looking after younger sister on their own in an apartment in San Myshuno - he has become her care dependent. He has the 'Successful Lineage' aspiration and as you can see from the screenshot, it's working, i.e. the game is recognising his sister as his 'child' and is fulfilling the requirements of the aspiration. Here's the shot - the girl is his younger sister:
You don't mention which aspiration you're doing, but it seems like this could be a bug? Long shot, but have you tried repairing your game? Or do you have any mods that might be conflicting? If not, might be worth setting up a bug report over at AHQ?