I swear that has caused more misunderstandings among Western viewers than anything else. Calling someone Big sister or brother, or martial brother in this case, does not imply a blood relation or any relation other than respect and familiarity.
I really like their relationship. It's two mature people who understand and respect each other. Honestly, that's more than some couples who are supposedly madly in love🙃
I agree with Ximena. They are like two friends who decided to get married. It makes sense as it's suitable for both of them. Love as essential for marriage is a very modern idea.
PS: based on his previous roles. Poor thing.