As jera has explained, the correct sentences are
Das sind meine Mutter and mein Vater
Das ist mein Tisch
That said, I would like to add something to help you understand why.
I think you are getting confused with the accusative because you are following a simple (and unfortunately misleading) rule: if it answers the question "Who(m)? What?" then it is accusative.
For example, let's take the sentence "I eat an apple". You can think: "I eat what? An apple". So "an apple" must be the object, and thus it's accusative, so the translation in German is: "Ich esse einen Apfel". "Einen" is clearly accusative, and it is correct. But here comes the tricky part: this doesn't work with the verb "to be".
With respect to "This is a table", if you reason like this you will think "This is what?", and since "A table" is the answer, you will conclude it must be accusative. Unfortunately, it's wrong!
The golden rule is that with the verb "to be" (sein) you never have the accusative, it's always nominative.
Thus, "Mutter" and "Vater" in that sentence are not the objects. They are "predicative nominals".