EDIT: It appears my page numbers were off, I thought it was about 300+ pages when I made my calculation, there was closer to 100 pages left. Also, my guess was pretty accurate.
and i'm even being nice and putting a spoiler tag on this because i'm that sure
i predict that we discover that his (as well as his father and brother's) illness is in fact the result of deliberate poisoning by a branch of the family that wishes to inherit and take over the property.
the poisons are taken from the exotic plants that grow in the marquess' conservatory
they are gathered by his female cousin who frequently comes by for clippings because of the exotic nature of the lovely plants. the cousin in question is who the family hoped he'd marry to fix the first between the branch, but who is also the sister to the man who will inherit the lands and title should the marquess die w/out a heir.
the poisons are then delivered by her maid to the marquess' valet, who is in on the whole thing and makes sure that the marquess gets his dose (likely mixed into his preferred alcohol)
some 183 pages to go, and i'm pretty damn sure that this is how it's going to fall out
probably some more drastic attempts on life/etc, maybe a dark secret concerning someone being wronged or slighted, and some profound declarations of love. oh yeah, the bride will probably be at least pregnant (if not a jump in time to show her now a mother) by the end of the story and they'll live happily ever after now that he's no longer being poisoned regulary.
though i have to say, out of this boxed set i'm reading for review, this is SO MUCH BETTER than the last few. plot is incredibly transparent to me, but the writing is decent, and as far as premises go, i've come across far worse.