do you think Ethan is justified in flirting with Mattie since he is a married man

I'd say yes. Ethan's wife is a nagging hypochondriac. There was no love between them. Mattie was the first warm person in his life in a very long time. It would have been unatural were Ethan not to be attracted to her.

Ethan is in love with her, which makes matters even worse. This is a hard call, Ethan is a man, men flirt....... so do women. Unfortunately, Ethan's flirting may be something more than harmless. Mattie is a distraction and somewhat of an obsession; reciprocation of these feelings is unacceptable.......... if he leads Mattie on, he is wrong, but all in all I can't blame him for being seduced by the youth and energy Mattie has brought into the house.