I feel like with the closeness of the big life events, this family has just gotten caught up in the ideas of registries and gift to the point that they aren't realizing that moving into a new home is not the sort of event that one registers for. I would decline or just bring a small, appropriate gift.
And, while in general it's not inappropriate to have a wedding shower and a baby shower and a housewarming party in one's lifetime, the GOH ought to keep in mind how often they are soliciting gifts (even with total politeness for each individual event) from the same group of people, especially if they are pretty substantial gifts. Three events with registries* containing mainly big-ticket items within a two-year span (if that's the situation) is a little much, cumulatively, even if each event is in itself reasonable. Especially considering that there's usually a large overlap in what would normally be given at a wedding shower and what is being specified for the housewarming party.
However, it's unclear to me how much the couple are "in on" the housewarming party invitation sent by her mother. I mean, one assumes they actually are
having a housewarming party at that time and date, or else they're going to be unpleasantly surprised when people start showing up! It seems odd to me that someone who doesn't live in the new house is the one inviting people to the housewarming party--that's more in line with how one would set up a wedding or baby shower, having someone other than the GOH be the host.
But, it could also have seemed totally innocent to the couple, like Mom is paying for the catering and helping decorate and so forth as a treat for her daughter and son-in-law, who after all are pretty busy with a new house and a small child. So it's possible the couple have no idea that this
was how Mom invited people, attaching their "list of stuff to buy after we get settled in the new house" and encouraging guests to purchase from it. I'm imagining their expressions as people appear at their party bearing matching towel sets, place settings, and small appliances--probably stunned gratitude morphing into horror as it dawns on them what Mom must have done, and how bad it could look for them. What would you even do
in that situation?
Or of course they could be totally in on it and are greedily asking for more major gifts from the same group of people, for an occasion that doesn't normally even have
Here's a question: If you as a guest got an invitation like the one Mom sent, and you want to know if/suspect that the couple have no idea that Mom is operating this way, how do you bring it to the couple's attention? Should
you? My first thought is to reply to the email and copy the couple as well, and say something like, "Hey, I was looking at the list of things you want for the housewarming party, and I wasn't sure if you wanted the towels in Navy or Black. Let me know!" If they write back, "Navy, to match our new bathroom," then you figure they're fine with what Mom did. But if they're like, "Um, please, do not feel the need to purchase anything on that list, just coming to the party would be great, thanks so much," then you might suspect they didn't know what Mom was up to before. Would this be a rude way to handle it?
*To me a list of specific items one wants guests to buy is a registry, or close enough.