It's all one sided, you don't know what he did or how he felt. This played out over months.

In her OP she even mentions that her husband gained weight but Liam is very fit (a lot of emphasis on fit).

You might think men are oblivious, but do you really think he didn't notice his wife eyefucking his brother all day?

She says they were fine before the pandemic so this started when Liam moved in, as in when she started drooling at the sight of another man who happened to be his brother. Of course he checked out and of course he started thinking that if she's like this when he's around, how on earth does she act around other men she finds attractive when he's not?