Can siblings marry?
I’m not aware of any government that will currently and knowingly marry full-blood siblings or recognize a marriage of full-blood siblings; rather, if it was discovered by the authorities after an official marriage was formed that the spouses were, in fact, siblings, the marriage would be dissolved and considered invalid. If the spouses knew they were siblings when they married, they would be subject to prosecution. If they discovered the genetic relationship after getting married, they would have to file for an annulment or dissolution or risk prosecution.
Where sibling consanguinamory isn’t still banned by law, siblings can have a wedding ceremony and live the married life, although under discrimination, as their government will not recognize their marriage and they will not get treated equally. Consulting a family law or estate attorney might be worth it for siblings who want some of the legal aspects of marriage. DO explain to the attorney of you genetic/legal relation as siblings. DO NOT tell the attorney you are lovers. Explain you would prefer each other to essentially be next of kin and reciprocal beneficiaries over and above anyone else (like your parents or another sibling); you want as much of a personal-social partnership as the law allows.