twins are hereditary through the woman's side. Typically they say it also skips a generation. You can't just have them unless you are going through fertility and have more then 1 embryo implanted and even then its not a for sure thing.
I agree you can't choose to have twins. It's nothing you can do. If it happens then so be but if not don't get sad. Be happy to get pregnant with just one. There are plenty of us out there having trouble trying to just to have one.
You can't control it. Twins are high risk. Be Happy if you can get pregnant.