The Great Big Problem is that while Gordon, Tracey, Elly, John, Connie, April and Jim know that Mike and Dee really got married in December, Liz does not. As far she she knows, their anniversary is in September and celebrates their freedom from the irrational rages of Mike's horrible mother in law. The reason why they didn't tell her relates to her angry reaction to Mike pretty much nailing it as to why she wanted to intrude on Anthony's first marriage. Just as she didn't like being told that she wanted to barge in like a vindictive idiot and call attention to herself and humiliate the evil French person who gave a clingy irritatant who thinks that the world owes her a living because she's cute her right name, she will feel all kinds of betrayed because no one saw fit to tell a chatty, easily offended annoyance a secret she simply couldn't have kept.
As you will recall, Liz hates Mike because he saw a truth she didn't want to face. What this tells me that Liz will hate the person who tells her the truth about herself for life and beyond. How dare this horrible person come along and tell her that she would certainly have been so enraged that something underhanded was going on that she would cause an argument that would take years to fix. How dare this person come along and say that no one trusted her to shut her mouth and how dare this horrible child who keeps her from being the baby bring up all the times when she shot off her fat yap. Rather than admit that she's a loose cannon who doesn't understand the delicacy of a self-imposed calamity, she's going to spend years trying to get April to admit that a loose-lipped idiot who can't shut her face can be trusted with secrets. This despite the fact that it's very likely that one of the first things she did when April graduated was to run her mouth about the time Gerald tried to get to second base while she was supposed to be babysitting.