The Yazidis are followers of one of the Middle East's minority religions, centred mainly around Sinjar in northern Iraq. They tend to be secretive about their beliefs and keep themselves to themselves: they don't accept converts and they don't marry non-Yazidis. Over the centuries they have often faced persecution, most recently at the hands of IS/Daesh which declared them to be devil-worshippers.

In 2014, IS forces captured Sinjar. Thousands of Yazidis fled to the mountains; others were killed or taken prisoner. Young women among the captives were often forced to convert to Islam and become sex slaves for IS fighters.

The result, now that IS has gone, is a religious and legal tangle. 

Among Yazidis, sex with a non-Yazidi is regarded as a sin and a reason for ostracising offenders, even in cases of rape. It's seen as a way of keeping their faith "pure".

Fortunately for women abducted by IS, however, the Yazidi Supreme Spiritual Council has decided to make an exception. This week it announced they will be accepted back into the community – along with children fathered by IS fighters. 

But there's another problem. In accordance with Islamic custom, Iraqi law says the religion of children is determined by the religion of their father. In other words, those born to Yazidi mothers as a result of rape or forced marriage with IS fighters must be officially registered as Muslims.

Registering everyone's religion is a common practice in Arab countries, and it's not a trivial matter. In the field of personal status law – marriage, divorce, inheritance, etc – different rules apply to members of different faiths.

This could cause lots of complications in adult life if the children of Yazidi mothers are legally regarded as Muslims. If the children are raised as Yazidis they will technically be converts from Islam – which the Yazidi religion doesn't allow. Legalistic-minded Muslims could also accuse them of apostasy for having "renounced" their father's religion – and many view apostasy as a sin that should be punished by death.

The Iraqi government seems to be taking a sympathetic view of the situation. A draft law introduced in parliament earlier this month will provide compensation and other forms of support for Yazidi women who survived the IS onslaught, but the existing law that says religion is transmitted to children through their father is unlikely to change. It's a deeply entrenched feature of the patriarchal system and also a way of ensuring that in a mixed marriage Islam is passed on to the next generation. If a Muslim man marries a non-Muslim woman the children automatically become Muslims. Muslim women, meanwhile, are not allowed to marry non-Muslim men.

This happens in other predominantly Muslim countries besides Iraq. It's especially problematic in Egypt where there is a large Christian minority. One example, described in a report by Human Rights Watch, involved Fadi Naguib Girgis who had been born into a Christian family in Alexandria. When he was five years old his father converted to Islam and left home. 

Fadi continued to live with his mother and regarded himself as Christian but when he applied for a national identity card officials claimed he was a Muslim, based on records which had been updated to show Islam as his father's religion. They accused him of forging his birth certificate and trying to convert from Islam to Christianity. He was then reported to the public prosecutor and detained until intervention by the Coptic pope secured his release.

• Gerard Russell's book about minority religions in the Middle East, "Heirs to Forgotten Kingdoms", has a chapter on the Yazidis.