My MIL wore a nice black pant suit I helped her pick out. She was not comfortable in a dress or skirt. I bought her a really pretty brooch and she looked great. My mother worse an ankle length skirt and a sparkly top. Each one was dressed in what made them most comfortable. It made them both happy, which in the end made me happy. I am sure you DD will want you to feel comfortable and happy with what you are wearing.
I would not worry about what the future MIL is going to wear, but if you wish to talk to her ahead of time you can. She may have a picture of her outfit, or at least an idea of what she is looking for. Just remember it is about your DD, and what she wants is all that matters.