Natalia,
In a word? Yes. It’s exceptionally tricky to organize, but it is possible to make your character related to another person’s character. It involves intense collaboration and it’s not something you can do without your writing partner’s permission, but I can think of several instances in which characters have shared back-story and/or genetics. Prime examples include:
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Rafael Lamaroux and
Simone Perry were half brother and sister.
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Rozaliya Donkova and
Julianna DiPallo were twin sisters.
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Matthieu Dumas and
Andre Dumas were brothers.
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Ashton Greene and
Lucian Michaud are daughter-in-law and father-in-law
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Lily Grace Hamilton and
Fredrick Worthington are sister and brother.
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Jean Sauveur and
Luc Danier are brothers.
If I were you, I’d ask any of the RPers who have written characters who are related to one another how they collaborated and if they have any pointers on creating shared histories. Also, be sure to explicitly discuss how you and your RP partner intend to work together. Personally, if I have shared history with another person’s character, I like to double (and triple) check whenever the situation allows. I’ve seen characters with shared histories work beautifully—but I’ve also seen it crumble and leave one member awkwardly hanging or bad blood between both RPers. Figure out each other’s styles and plans before you do anything drastic, and I guarantee you, it’ll be smooth sailing.
Good luck!