They announced over Christmas that they were married already but would be having the wedding in the fall.
This your BIL & his wife. They sound like courteous, kind people with their hearts in the right place. They kept things quiet to keep the focus on Lord L & you during your special time.
Is the way they are doing things proper from a strict etiquette standpoint? No, no and no
Do you want to use this opportunity to "teach" them proper etiquette or make a point? Don't. Wait until you have received the invitation for ceremony in the fall & buy whatever price range gift you would normally give.
There's a saying around here "safety trumps etiquette."
A good rule to remember, especially when dealing with non-toxic family is "kindness trumps etiquette."
I agree with this, especially the bolded.
Very strictly speaking, of course, what they will be doing is not a wedding - it's a blessing, or a vows renewal, or a celebration of their marriage, or a religious ceremony, or a combination if the above - But for them it is a wedding, they aren't trying to double-dip or gift grab, so I think the kind and appropriate thing to do is to treat it exactly as you would have done had they not felt they needed to elope to sort out the legalities - send or bring them a gift at the celebration of their marriage. If they are people you would normally buy a housewarming gift for, buy them a housewarming gift. If not, not.
As something of a pedant, it would grate a little for me that hey were describing it as a wedding, but I see that as my issue, not theirs.
My cousin had a similar situation - her husband is from overseas. There was a very sudden change about the rules for job applicants in his particular field and the stage of career he was at which meant that if he were married, he would be able to continue with all of his job applications as planned. If not, due to the rule change, he would have to leave the country, re-apply for a different type of visa, and then return and reapply for all the jobs (in practice, meaning he would almost certainly lose 12 months worth of seniority as the chances of getting the appropriate visa in place before the closing dates for the jobs was virtually non-existant)
Their wedding had been planned for *after* they'd both finished all their exams etc, lots of his family were travelling to the country for the wedding, and so on. Their solution was to speak with their priest who carried out a very private marriage ceremony with just the two of them and 2 witnesses, and the wedding went ahead on the original date. It took place in church and with the same priest officiating, so presumably he hd no problem with going through the ceremony a second time even tough it was not strictly necessary. No one else learned about the first ceremony until about a year later.