They are soulmates, there's no way to explain it, they are simply made to be.
Bestfriends for life and that's a fact, always have each others back and would kill to protect them and even hide a body.
They are the first soulmates in the world, also the first ones to ever prove that you don't need romance to have a soulmate.
"Oh! Look is them again, Luiza and Maria, always together."