98 I don't think becoming blood sisters/brothers is messed up, that isn't to say that I would ever become one or advice people to become one. In fact the opposite of that. This however is more because sharing blood in a way like Mahlah and Raven did (and the way I presume you read most people do it) isn't exactly safe. In this case, for example Mahlah could have easily ended up with being infected by those parasites herself assuming they can be in Raven's blood.
The act itself is in my eyes actually rather cute (This isn't the word I want to use for it but I lack knowledge of a better one). The fact that you need to cut yourself shows that you are willing to go through pain for the other and then there is also the fact that blood is often associated with life itself which means that after the act you have some of your friends life inside of you and the other way around. (Though, most of the time your immune system is going to put a hold to this sooner rather then later. Stupid Immune system
)
I actually also happen to know what shaped my opinion on this matter to this. I read a story once of child been bullied because he was bold and often stayed home because he was sick. The main character of the story became friends with the bold child and eventually blood brothers. Shortly after that it was revealed that the bold child had cancer and a few days later (or maybe even less) died because of it.
However, the now dead child becomes a spirit only visible for the main character. Which, due to main character not knowing he had become a spirit causes weird situations for him and eventually even the spirit needs to leave.
I sadly forgot a good amount of details from this story, even its name though I suspect it is called "blood brothers" or something along those lines. This is not because it is a bad story but more because of how long ago it was since I read it.