When I got married I told my matron of honor to do this - just buy something she liked that was formal enough to wear to the wedding. I didn’t have bridesmaids because I’ve always thought it was a waste, really - ask someone to pay for a dress just so they could stand up at the front of the church for 30 minutes.
The 2 bridesmaid/maid of honor dresses I had to buy went to Goodwill not too long after those respective events.
I own several bridesmaids dresses, in a variety of sizes. One I really liked and did wear it again. In one wedding I was told to just wear something pink, which resulted in the three bridesmaids wearing three wildly different shades of pink and looking like we just happened to meet up for a party. 
At my own wedding, my three attendants looked gorgeous in their dresses which they each loved but one of them left hers at my house. She had just had a baby and said she never intended to be that size again. I doubt any of them wore the dress again but it wasn’t all that expensive.
My closets also hold four prom dresses that will never again see the light of day. Fashions have changed quite a bit from even just a few years ago!
I was only in one wedding, my cousin’s, and I have no idea what happened to that yellow dress. All of us bridesmaids met at the wedding dress shop and were asked to vote on 3 different dresses. These were the days of matching dresses. The one I wanted came in first but my aunt, bride’s mother, vetoed it because at least one of the bridesmaids was very busty so my aunt wanted one with plenty of coverage.
My Mom was in 8 weddings. The dresses were in a trunk in the upstairs of my Grandmother’s farmhouse. My sister and I would go up there and dress up. Then we were allowed to bring them home and we played dress up in the basement. Fun times.
How old are those prom dresses? If they’re still in good shape, maybe the local community theater would like to have them?
I recall a wedding I was involved in in some role between guest and groom. I forget which one. But the relevant point here was the matching bridesmaid dresses in monochromatic satin had a square and rather low scoop neckline (I don’t know the proper term). Low at least by the standards of the era. Three of the women had little inserts made from leftover hem material and sewn into the bottom of the scoop to hide more of their cleavage / breastage.
It was one of those things that looked fine from across the room, but once seen face to face could not be unseen. The sewing itself was adequate; this wasn’t a butcher job. But boy howdy was it obvious once you noticed it or had it pointed out to you. A vast expanse of seamless satin covering the torso plus this rectangular plug 8" wide and 1 to 3" high. Tee hee.
When my son and his bride married, she wore my wife’s wedding dress. She had to take it in a bit here and out a bit there, but it was nice to see that dress again!
I’m in the process of packing to move across country and came across the bridesmaid dress I wore over twenty years ago. The bride was a great friend who only asked that we picked our dresses with her colors. My dress was a very nice peach colored sateen knee-length short-sleeved dress that I could wear almost everywhere.
I should have turned it into toss pillows after I outgrew it, but now it will go into the trash because it shows the wear and can’t be donated.
That’s so sweet! Especially that the new bride was willing. Bodes well for your merged families.