Why god allowed to marry brother and sister?

Please explain what you mean.

Your question is not clear, thanks.

Was it God, or the parents?

Why god allowed to marry brother and sister?
I don't really have quite much information about sunnies (I'm shia) , but it's a great sin because sister is a Mahram person like mother and aunts , and the god never allowed this marriage , looks like you never read quran before... did you know about "Loot tribe" and why god punish them ? You can find all of answer in quran JUST READ IT CAREFULLY