1.) In this book, [it] is the father who tries to murder her.
2.) In this book, [it] is the father and John who try to murder her.
3.) In this book, [it] is her parents who try to murder her.
Your question involves the it-cleft construction.
Basically, what is happening is that a simpler sentence has been cleaved into two. One part of it (e.g. "the father") has been foregrounded into a main clause, while the rest has been backgrounded as a relative clause.
- a.) The father tries to murder her.
The info "The father" can be foregrounded, via the main clause in an it-cleft construction,
- b.) [It is the father] [who tries to murder her].
The main clause form of "It BE X" is fixed, for the most part. Usually it is of the form "It is X", "It was X", "It will be X", etc.
The rest of the info is in the form of a relative clause. Note that this is a slightly different type of (integrated) relative clause from the other types that are often taught in school and in grammar books (which are the usual integrated relatives and the supplementary relatives).
For your #2 version,
2.a) In this book, [the father and John] try to murder her.
2.b) In this book, [it is [the father and John]] [who try to murder her].
And your #3 version is similar.