Juliee, Congrats on your daughter's wedding!
You may be able to find separates, however dresses can work well (you might need to buy a size larger, then alter. The large department stores will do it for you for a cost and let you know if it is possible to alter).
For an outdoor wedding in the summer I recommend a short dress vs. a pantsuit or long dress. You are going to be stressed enough, plus the hot weather-don't wear more fabric than necessary-especially since you have nice legs ;)
Here are a few dress suggestions for an "E" mother-of-the-bride...
https://shop.nordstrom.com/s/alex-evenings-embellished-surplice-sheath-dress-plus-size/4234486?origin=category-personalizedsort&breadcrumb=Home%2FWomen%2FClothing%2FPlus-Size%20Clothing%2FDresses&color=fuchsia
https://www.dillards.com/p/adrianna-papell-plus-size-pleated-foil-sleeveless-midi-dress/508820434
https://www.dillards.com/p/tahari-asl-plus-size-lace-side-tie-sleeveless-fit--flare-dress/508750310
https://www.dillards.com/p/alex-evenings-plus-cold-shoulder-popover-dress/506869451
I LOVE this one below (4 colors)!
https://www.dillards.com/p/s.l.-fashions-plus-georgette-tiered-short-sleeve-capelet-dress/506706425
Let us know what you choose. Perhaps share a photo of your mother of the bride dress?
Love, Leslie MSP