Now, this is mainly just speculation on my part without reading the descriptions.
Losing a love interest seems more tied to losing your will
to live on. A love interest is an equal. Maybe not an equal in combat, but a mental equal. It's someone who's tied to your wants and desires. If someone like that dies, you don't think you can live on.
Losing a little sister is more akin to losing your right
to live on. A little sister isn't an equal, but someone to protect. Someone you have an obligation to protect. Normally you can live on, but since you failed, you don't think you deserve it.
You don't have the same kind of relationships to the two types, and subsequently the emotions aren't the same. I'm however not sure how much practical difference it makes, though.