姑 (しゅうとめ, 'mother-in-law') is the specific term. 気にさわる ('to get on someone's nerves') is the idiom. ~なければよかったと uses the regret pattern ('if only she hadn't'). 臍を噛む (ほぞをかむ, 'to bite one's navel') is a literary idiom meaning 'to regret bitterly' — physically impossible, hence the intensity of the metaphor. The sentence layers two idioms within a complex conditional-regret structure.