I love your idea, and I think you will be a great mother in law, becasue you already love her so much, and want to show how special you think she is. Keep that attitude, and you should have a great, loving relationship for years to come.
I am afraid I agree that to make the dress may be quite an undertaking, and not to discourage you, but satins or silks are very slippery to work with for experienced quilters or seamstresses.
And I wonder if the Butterick pattern would work with that type of fabric, it seems to want heavier fabric (denim, linen). That might mean that a different hang or something if a different fabric is used.
If your daughter in law to be says she has other plans for a dress, I have a few suggestions, that would still allow you to use your dresses and allow you to make something special for her.
1- if she is having a dress made, ask if maybe the large sections of your skirts, could be used on the bodice of her gown. That way it is still your dresses, and still her own too.
2- if she is buying a ready made, depending on the style, perhaps you could have a belt/bow made to attach.
3- make a short train from your 2 dresses, which can attach/detach from her dress. She could have a train for the wedding, then remove it for the reception.
4- ask the dressmaker/bridal shop for some of the fabric used in her purchased dress, to add to what you have. Then anything you do, will coordinate.
5- make a shawl for her shoulders or a simple shrug (as so many dresses are sleeveless), so she will stay warm after the ceremony
6- make a ring bearer pillow
7- make a photo album using the fabrics from your dresses
And most importantly - do not be discouraged. We are trying to help, to be sure that you see the down side. But, we all had first projects, and some of the first projects that have been posted here have been amazing!
Good luck and let us know what you do, we want to see pics!