I would not be a part of this fiasco but neither would my DH, so....

This is so backwards... hiding the fact that their child is LEGITIMATE! Many people have a celebration well after the actual marriage. It sounds like there are other issues, but if this is the only one I would go and pretend it is a celebration of their original marriage. 

I'm unclear on why you don't want to go. Maybe you're unclear on why you don't want to go too. But you need to choose a valid excuse and stick with it.

You don't want to go because it's a lie?

I get that. When everyone finds out, they might not like that a chosen few were in on the secret. But you are not likely to be the target if that happens -- after all, you're "just the SM!" That target will likely be on the back of your husband and the happy couple. And anyway, you're still in on the lie because you went to the first wedding and never told anyone. I'd skip this excuse.

You mentioned that you and SD don't get along. If that's why you don't want to go, why did you go to the first wedding? I'd skip this excuse too.

It sounds as if you don't want to go for another reason -- like maybe you don't want to be around BM? Perfectly understandable. That is your most legitimate excuse.

I don't think you have to go. But be sure you choose an excuse that is valid and honest because you probably have a good one. But if you throw in other excuses that don't ring true, your husband will shoot holes in them and you'll be left with no defense -- you'll just sound like you're back peddling.

The whole situation sounds silly. My sister uses info to control people -- I constantly have to remember what I am not supposed to say or know when I'm around my mother and what my sister deemed "top secret." It's a control thing.

I wouldn't go. DH's son got married at city hall, and then had a "wedding" 6 months later. I told DH I wasn't going, because they were already married. DH ended up not going, because he didn't feel like being around BM. 

Going to the reception is a good compromise if your husband needs support. I'd go for the wedding cake.

I wouldn't want to go, either. The entire situation feels dishonest to me.

An already-married-with-a-child couple has no need of a wedding and reception. A vow renewal with no presents expected would be fun, but this just sounds like a gift grab. And who's paying for it? That money would be better off in a college fund for the couple's child.

I was legally married in a courthouse ceremony because DH wanted that commitment and stability for me. He was returning to work in Afghanistan and wanted a layer of legal protection for us that being married gave us. Here we are, 8 years in, and still need to have a "real" wedding with a rabbi and friends and cake. Is my religious wedding dishonest? No, its just another part of our journey together. We will get to it when we want to.

I'm assuming SD and her DH got married for the baby? Then kept it quiet to keep from hearing the drama from mothers of the bride and groom who want the wedding to celebrate themselves? There is no fury like a MOtB who has her thunder stolen by a civil ceremony. 

Even though SD is a wretched beast to you, maybe cut her some slack in this? She did trust you with her secret and might be depending on you to help smooth things over.

I think you should go, for your DH.

If I had to take a guess, I'd say they got married for health insurance and to "legitimatize" the child. Them being legally wed doesn't mean they don't want to have a wedding. It doesn't mean that it is a gift grab (and, actually, after 2 years married and keeping silent about it, I'm less inclined to think that because gift-grabby people would have wanted gifts for their "legal" wedding and their "ceremonial" one). It doesn't mean that that they shouldn't have one.

Unless SD has been overtly toxic, cruel, or violent, I think you suck it up and go. We all make sacrifices for our spouses, and that includes attending family events we don't want to go to. You don't, as of yet, have a solid reason why going would be detrimental to you. I can guarantee you, though, that NOT going with the shabby excuse of "it's all a lie!" will be detrimental to your marriage.

I think the best compromise in all of this would be for you to keep the car keys. If things get to be too much, leave. Just don't put your DH in the position of feeling like he has to choose when it comes to you or his kid when his kid isn't actually doing anything wrong. Save your "NO" for when it really counts.

I think you should go. Obviously SD trusts you more than her mother. Be there to watch SD squirm and worry that you might spill the beans. Since she doesn’t even acknowledge you this would be an awesome opportunity to play her game at her own wedding. Go and put your nose in the air and let her little secret accidentally slip out because you had one too many to drink lol Or go to witness her sickness and give her all the knowing glance every so often at the wedding. I’m just being facetious of course.

IMHO you should follow your gut and stay true to you. I would go to share the occasion with the man I love.

I guess I'm split. On one hand, OSD trusts you enough to invite you to the wedding, in her shoes, if I didn't trust you I probably wouldn't have invited dad either.

I have two friends that had a secret wedding, were married for almost 2 years, no one buy myself a one or two others knew, it was what it was to me. I respected their decision and it wasn't my place ot judge because I'm sure there was more going on than I saw. They ended up divorced, but that being besides the point...

My DH and I got married COMPLETELY alone, just me and him in the courthouse. We didn't want either side to feel they had to travel for it, we really don't believe that they have the time and resources to do so and since it was 2500 miles apart, we chose to celebrate ourselves rather than place that kind of a burden on them. We do however intend to do some kind of celebration in the future when we can afford to do one in both locations It won't be a wedding though, probably more of a "vow renewal" or a party or something. lol

So i get wanting to celebrate with friends and family. And I really believe that's partially for them too. My Grandma just last week was trying to convince my mom they should throw a belated bridal shower for me. I've been married for a while now... But she just wants to be part of it I guess.

It does sound a bit deceitful, but maybe it's just to help save some people's feelings. I don't know. 

Back to the original topic though, lol. Weddings are simple, if you want to go then go, if you don't then don't. Don't let anyone pressure you into it out of "necessity." I think just the reception is a good compromise if that's all you feel comfortable doing. However I'd probably just go to spend the time with DH. It might be their day, but your SO is the dad, so the celebration is kind of for him too, he just wants to be there to share his little girl's big day, and I'm sure h'ed love to have his person by his side too