There is a book in the National Library of Australia with a ridiculously long title: A new methodical instruction book, for the flute [microform] : containing in two distinct parts, a comprehensive elementary exposition, elucidated by proper examples, with a finishing section, in which are displayed, all the beauties of which this instrument is capable. It is written with great perspicuity, that it may be intelligible to the meanest capacity & will lead the pupil through a progressive & pleasing course of instruction, to the highest pitch of excellence, in a very short time. The whole composed in an original style, & dedicated to Mr. Willoughby G. Weiss by his brother, Chas. N. Weiss
So, Charles Nicholas Weiss had a brother called Willoughby. A Google search of "Willoughby Weiss" comes up with Willoughby Hunter Weiss, an English opera singer. However, his father was Willoughby Gaspard Weiss, a professor of flute and a music publisher. I'd say this was Charles' brother.
The Morning Chronicle of 30 March 1853 has the following death notice: "On the 25th instant, at the residence of his son-in-law, E. Edwards, surgeon, Crew, of bronchitis, aged 70, Willoughby Gaspard Weiss, Esq., of St. Clement’s-terrace, Liverpool." This would have made his year of birth 1783. This fits with the vague idea of when Charles' date of birth was.
So, Charles had a brother called Willoughby Gaspard Weiss. Gaspard seems to have been a family name in my branch of the Weiss family, but Willoughby is an unusually British sounding name for someone presumably born (like his brother Charles) in Mulhouse, France/Germany. I was quite bewildered by his name until this afternoon, when I put two and two together: Charles and Willoughby's father Carl was known to have taught the amateur flautist Lord Abingdon. This was the 4th Earl of Abingdon, whose name was Willoughby Bertie (1740-1799) - and I think this solves the mystery of Charles' brother's very British-sounding name.