i love these types of fanfics. 
i know they're popular and some say that they're overused, but i love them. 
a gilbert or a salvatore falls in love with a mikaelson? i will read it every time i see it. i'm a sucker for that type of plot. 
when the oc is forced to choose between their family and their love interest? when they just runaway together? you can say what you want, i love these fanfics.
which is why i wrote one.
and i'm writing another.
so here are some tips on how to write them.
1. remember that you are changing a family dynamic.
you are adding a whole new character, a new member of the family. 
it will change. it has to. the only exception i can think of is if they're the oldest sibling and were sent away and never mentioned again.
then this doesn't really apply (though it will once they meet i guess).
the relationship between siblings could drastically changed and don't be afraid to do this.
for example, if you add a sibling of the salvatore who is really close to both damon and stefan and protecting them is the only thing they can agree upon, you might have a completely different relationship between stefan and damon. maybe they never fall out and always stay together to keep their other sibling safe. 
don't be afraid to let things change. it's realistic. it's not "writing them out of character". there is a whole new person that is going to be with them from when they're very young or even before they're born if they're older. this could even change the relationship between them and their parents. it's a big difference. don't be afraid to let it affect the family dynamic.
2. if you don't want a new oc to change the family dynamic, you have to be creative. you have to find a way to make things as they were without your oc.
this is a bit of a spoiler for a fanfic i'm writing where the oc is stefan and damon's sister. similar to the example i gave above, they both love and want to protect her, so much show that if she was there when they turned, they would try to work together for her sake. but she's not there, she's taken by katherine. therefore, their relationship is fine to go back to how it was in tvd, with some minor exceptions. 
3. use examples from the shows to help!
many of the characters in the shows are siblings. elena, stefan, damon, jeremy, all tbe mikaelsons, the parkers. what better way to start developing the relationship between your oc and their sibling then by looking to the show for example?
now of course some things will be different because the relationship between someone and their siblings is not the same for each one of their siblings, but you can pull from what is already written.
for example, if your oc is a mikaelson sibling, you have a lot to work with. look at how they treat each other and apply that to your oc, altering it to fit the personality of your oc and the dynamic that you want to have. 
                                      
                                   
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the vampire diaries fanfic writing tips
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