What does 我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当 mean? (from 活着, p.3) I'm having trouble understanding this:


  他手指挖着裤管上的泥巴，愤怒地告诉我是他那不孝的儿子，当我再问为何打他时，他支支吾吾说不清楚了，我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当。
  活着, p.3


My translation is:


  [With] his finger picking trouser-leg mud, [he] angrily told me that it was his unfilial son, [but] when I next asked him why hit him, he hummed-and-hawed [and] spoke unclearly; I immediately knew he (准是对 ???) [his] daughter-in-law work (?) [in] stealthy, shady business.


Question: What does 我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当 mean?

It seems like 准 means allow, but I don't know why 是 nor 对 is used here.  It's possible 干 may mean "to have sex" here.
我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当. 


准 means surely;definitely.

是 is used as an adverb:


  （重读， 表示坚决肯定， 含有“的确、实在”的意思）


E.g. 他是不知道 // He certainly doesn't know.

对 is a preposition, meaning to. E.g. 你对我做了什么 // What have you done to me.

干 means 做(do). 

Over all, the sentence roughly means: 


   I immediately knew he surely had done some stealthy, shady business to his daughter-in-law. 


P.S. As you suspect, the sentence might invoke some sexual affairs.