Eric - that's possible, and surely on some level, true. On the other hand, your brother-in-law, is suggesting that there is a way beyond the Heisenberg Uncertainty Principle, in which any measuring device we use to determine the nature of an entity effects the nature of the entity. (Which we may be seeing here regarding all these definitions of aiki).
Regarding Ueshiba, however, I don't quite see how what your bro-in-law states translates - the limit of a simile is that it is bounded by the degree of similitude.
Are you referring to the observor effect? That isn't quite what the bro-in-law was getting at. He was saying that the way particles and waves are defined mean they are insufficient to explain light. It isn't that light is both a particle and a wave, it is neither, and the concepts of particle and wave are insufficient. They work well when talking about sound, ocean waves, molecules of water, dust, but they don't work well for light.
I see that as being a worthwhile point to consider. Fire-water, heaven-earth, dual spirals, in-yo, Izanagi-Izanumi, triangle-square-circle, kototama, shihonage, push tests, reverse breathing, none of these are sufficient to explain what Ueshiba meant by aiki. Each might explain some tiny facet, but adding them all together doesn't equal aiki any more than particle plus wave equals light. That means any time he uses these terms, they will be inexact representations of aiki. So, my line of thinking is that it is likely he recognized the inconsistencies in what he was saying, but there wasn't really a good way around it.
But, I could be wrong.