Hmmm
Few things you ( maybe your bro) must take into consideration
1- a baby will be only consider legal child of a married couple if officially registered as husband.
2- to get a passport, a child must born from a registered married parents.
3-If anything happens to the parents, the child only able to get the Insurance & Retirement money if the parents are registered as married couple.
4- When it comes to government application on any housing loan or maybe the child education in top universities must comes from an officially married couple.