A joke involving 忆苦思甜 I ran into the following wordplay: 

That's your dad! [...] I heard he has two wives, one back in the countryside,
and a new one. Why do you think one's called Sitian and the other is called Yiku?


I did some research on Google and found this idiom: 忆苦思甜。I thought this was somewhat relevant here, but I'm still having a hard time understanding the joke. How would you render it in English?
Why this joke is not funny? Because it doesn't have any twist and turn. And the so call wordplay is too straight forward to be called wordplay

The man's wife in the countryside is named Yiku (忆苦 - remembering hardship) and the man's other wife in the city is named Sitian (思甜 - think of the sweet (life))

Most people would deduct that a couple named their daughter 忆苦 to remind her the hard life they endured as a family when she was young. And can't help but sympathize with her. 

In short, the joke is stating 'country life is tough, city life is sweet'