Captain Tilney, when flirting with Isabella in Catherine's presence, laments `What! Always to be watched, in person or by proxy!'

OK, Catherine is the proxy, but who is she proxy for? On first reading it seems clear that she is proxy for her brother James, standing guard over Isabella, but am I imagining things by thinking that Captain Tilney is also seeing her as proxy for his brother Henry, and feeling that she's watching over him rather than Isabella? Given the tangled relationships between the TIlneys, Thorpes and Morlands that lead Isabella to say `There are more ways than one that we can be sisters', am I being fanciful here?