What did you make of the kiss Sonia gives her sister-in-law?
It seemed to me, based on her later statement to her brother-in-law about how ugly men are, and the unfeeling sexual behavior of her husband, that she was searching for some way to express her sexual feelings.
I think that since she had just had a baby and maybe being of a certain age too - she was a ticking bomb of hormones. She had no way to release them, thanks to her husband not really connecting with her in the bedroom.
So, as a result any touch at all, any contact seemed to set her off (putting on a shirt, breastfeeding, the sister-in-law rubbing her shoulders and back...). She mentions even as a child having a fire inside of her when she meets with the elder guy. What she's saying is almost like what sex addicts say about their compulsion. She feels that same overwhelming torment but she doesn't often have the opportunity to act on it.