Which is appropriate while addressing in-laws?
My mother-in-law and father-in-law are visiting us this weekend.
My mother and father-in-law are visiting us this weekend.
The first avoids the ambiguity of the second, where the visitors could conceivably be the speaker's father-in-law and the speaker's mother.
As Barrie has said, one removes the ambiguity you have identified.
You could say something more idiomatic, like "My in-laws are coming," or "My wife's parents are coming," which are certainly less awkward. Or even, "My wife's mother and father are coming," if you want to include both separately.
If you are writing the sentence, punctuating it this way will help the meaning, too:
My mother- and father-in-law are visiting us this weekend.
It's called a suspended hyphen. If you're speaking it, though, it's probably better to say mother-in-law and father-in-law.
My parents-in-law are visiting us this weekend.