First, although Arthur's lineage is quite dubious, regardless, in the eyes of the Marquis of Hastings, his distant cousin Sir Arthur Hastings is a legitimate lineage of the Earl of Huntingdon.
Secondly, although Arthur falls far short of the middle-class standards in the eyes of the Earl of Dalmo, in this little patch of London, Sir Arthur Hastings might not be considered a big deal, but at the very least, he's a small deal. Therefore, he doesn't have any financial concerns.
More importantly, he seems to not care much about things like dowries.
If he truly likes Flora, perhaps in a moment of happiness, he could stomp his foot and want nothing in terms of dowry.
Of course, even if Arthur doesn't want anything, the Marquis of Hastings would still provide his sister with some property, because he still wants to mingle within the British nobility circle. If word gets out that the Hastings family sends off a daughter without a penny, then what would happen to the family's reputation?
