I was able to find signature examples of both John and Margaret. All examples of Margaret's signature include her initials, M.M. The only example of John's signature that I have yet found is from an oil painting, but the form of the letters, especially the G, strongly resembles the signature of this little watercolor, and indicates that it may more likely be by his hand than by hers.
Margaret Malpass Geiszel
John H. Geiszell
The watercolor in question...
Taking into account that the different media have to be handled differently, I think you'll agree that the similarity is clearly seen. Of course, what's also clearly seen is the similarity between Margaret's and John's signatures... So at this point, it's still an attribution, but I'm prepared to attribute it to John.