Hi Anthony,
Alaska really is a toss of the dice, weather-wise. We went at the end of July and had outstanding weather each and every day, with the exception of a very light rain in Juneau, which lasted about 20 minutes. I've read that Ketchikan can be very rainy and we went prepared for rain. However, we lucked out and the sun was in full sight all day.
We did have some motion, primarily when going from Vancouver through the Inside Passage. However, nothing awful. If your mother in law is prone to motion sickness, there are so many things she can do to alleviate that problem, as discussed on many posts, previously.
You will all love Alaska. It's an amazing cruise showcasing awesome scenery and experiences.