Qatar's Family Law outlines clear prohibitions on who a person may marry, primarily based on family relationships. Article 20 prohibits marriage on the grounds of consanguinity (blood relations), barring marriage to direct ascendants (such as parents and grandparents, however far up the family line), direct descendants (children, grandchildren, and so on), and descendants of either or both parents, such as siblings, nephews, and nieces.
Beyond blood relations, Article 6 prohibits engagement — and by extension marriage — to women who are permanently or temporarily forbidden under Islamic law. However, the law does permit a subtle allusion to future engagement while a woman is observing her Iddat (the waiting period following divorce or the death of a spouse), though no formal engagement can take place during this time.
For expats, these prohibitions are relevant if you are seeking to register or authenticate a marriage in Qatar. Even if a marriage between certain relatives might be legally permissible in your home country, Qatari authorities will not authenticate a marriage that violates these provisions. If you are unsure whether your intended marriage could be subject to any prohibition under Qatari law, consulting a local family law attorney before proceeding is strongly recommended.
This is general legal information, not legal advice. For advice on your specific situation, consult a lawyer licensed in Qatar.