Personally first thing I think of is the detergent and I don't think it is very usable. But there are other name options that start with Cass that sound good with Isabel ... Cassia, Cassandra, Cassidy, Cassiopeia, Cassara, and Cassiana.
I actually saw a profile of a girl named Cascade in the newspaper! I think it makes a great middle name as well. IsabelCascade is pretty. Honestly, my first thought is waterfalls when I hear it. Then the detergent. So I think it works.
I think of the detergent, but I also think of waterfalls and the mountain range. It has a pretty sound. Cascada in Spanish (rhymes with nada, not Ada) might get you some distance from the detergent if you like it. Cascada is the name of a German band though FWIW.