They are in their houses = Onlar evlerindeler.

evet doğrudur ,ev+de+ler =they are in the house =in the =de , ler= (they) are

Onlar evlerindeler.

Well, this seems somehow difficult to me. Suffixes would be : plural + person + gramm case + verb “to be”, being “ev - ler - leri - de - ler”, is this the correct formula ?

Please explain or suggest your versions. So far the suggested phrase is absolutely normal.