at this point what other sign indicates zeena suspicious of ethans feelings about mattie

Ethan seems happy around Mattie. He helps her with housework. He likes to be alone with her.

The following quote from Chapter One seems to mention the change in Zeena's attitude for the first time.

"His wife had never shown any jealousy of Mattie, but of late she had grumbled increasingly over the house-work and found oblique ways of attracting attention to the girl's inefficiency."