Ben Wong and Crystal Tin play a married couple while Alice Chan and Lawerence Yan are also a married couple. I watched this years ago when it came out. Derek Yee or To plays Ben's younger brother. Alice and Lawerence own a lingerie company but Lawerence is unfaithful and I think Ben is a gambler. I don't remember much but I remember somehow they switched spouses in the end? Or not. I remember enjoying the story and ending.
Oh yeah, Ben, I believe plays a lawyer. I think by the end of the series, the couples appreciate each other more than at the beginning of the series.
Creidesca saw this series too, maybe if she's around, she'll be able to answer your questions better.
If you can read Chinese, I think ATV's official page for the series can tell you something:
http://www.hkatv.com/drama/gamble/