My personal opinion is do not go out of your way to reach out to congratulate the family member. If you were in a situation in which you and that person were present and the topic came up a mannerly "congrats" would be appropriate, but there's no need to artificially reach out to someone you know has been deceitful.
I tried to "play nice" / "pretend nothing was going on" with my ExW and her family during IHS and a lot of good that did. Looking back it would've probably been better just to be straightforward and honest about the situation.
