Several instances have been shown to show her attempt to accept Simran gracefully.Her resentment started only when Simran's interference(absolutely unintended)started affecting her personal life with Abhi and mainly Simran's behavior in her bedroom.
I'm not saying it's impossible but I can't imagine any wife not feeling a little uneasy.
Abhi was never very patient with her...it's a fact that he never understood her but he expected Neha to understand his feelings for her.
It was not anybody's fault because nobody was doing any harm intentionally.Both the women were feeling insecure.
I found these situations very close to real life because many times we hurt people's feeling without meaning to or feel insecure and suffer silently.
I absolutely understand why some people feel that Neha was unduly insecure because I'm sure there are women who are more confident and assertive and can see the whole thing as a woman's weakness.