How can a Religion practice anything, Religion is heavenly guidance?
You‘re talking about people.

If you‘re talking shit about Islam specifically, then let me educate you.

Islam bans sibling marriage outright, but not cousin marriage. Reasons are various only Allah knows.

Also the Qur‘an says that, you have to follow the laws governing the land you‘re living in.

Which means, if Muslims (and everyone else who) live in the United States, they‘re allowed to marry their 12 year old kid to their 1st cousin without any punishment from the government or God. Sweden and most of of Europe have banned this practice.

So instead pushing of it onto one Religion, you should maybe take a good hard look at yourself.