Are Goneril and Reagan justified in treating their father the way they do in "King Lear"?
That is for you to decide. Consider the evidence:
Goneril and Reagan go on and on in the first Act, prostrating themselves in front of their father and espousing on their undying love and respect for him. To then treat him with disrespect - ignoring his requests, pushing him aside, etc. - after he passes on his kingdom is hypocritical.
Take Goneril as the example for the moment. In Act I, scene i, she proclaims her undying love for her father:
Sir, I love you more than words can wield the matter;
Dearer than eyesight, space, and liberty;
Beyond what can be valued, rich or rare;
However, in scene iii, she is already taking about how she is annoyed by him and doesn't want to be in his presence:
When he returns from hunting,
I will not speak with him; say I am sick:
On the other hand, there is some suggestion that Lear doles out harsh treatment, and therefore deserves the same. In the example above, Goneril's annoyance has been ignited by Lear's treatment of her servants:
GONERIL: Did my father strike my gentleman for chiding of
OSWALD: Yes, madam.
Remember, also, that Lear demanded shows of affection from his daughters, although fathers are supposed to love their daughters unconditionally.
So there are argument on both sides - you have decide if Lear's flaws justify his daughters' behaviors, or if father and daughters are both flawed and all guilty.