Question

Ronnie is married......my cousin.

Answer 1

Ronnie is married  ___with___ my cousin

Answer 2

To

Ronnie is married to my cousin.

To is applied as an infinitive marker here in this sentence. It is used to indicate that the verb is in the infinitive while using with the base form the said verb.

To here is used to show the relationship between the primary and the second subject in the sentence here, where the primary subject is ‘Ronnie’ and the second subject being ‘cousin’.