I've seen March 1798 as the "setting" for this part of NA. Frederick's regiment (12th LD) was in Ireland up until 1793, when it returned to England. It then went to Toulon, acting as dismounted Dragoons (one company was sent to Corsica), coming back in 1795. The 12th went to Portugual with General Stuart to defend Lisbon, c1797-1800, (Which opens conjecture as to why Frederick is not with them.)

Thus, while Frederick may still have an affectionate relationship with Eleanor, he may well have been away for several years. It is no wonder that she is closer to Henry, even given differences in character and temperment. (Don't most siblings have "preferred" companions?) It might be worth figuring out all their relative ages as to when they'd have been together or apart. (Any takers? I'll be busy the next little while.)