Tony Lumpkin is Mrs. Hardcastle's son from her first marriage. Constance Neville Tony's cousin who lives with her aunt— Mrs. Hardcastle. Tony and Constance are being manipulated into marriage by Mrs. Hardcastle whose interest is in retaining Constance's jewels in the family. However, both Tony and Constance have other ideas. Tony wishes to marry his girlfriend "big Betty Bouncer" while Constance refuses to be separated from Hastings, her admirer. Mrs. Hardcastle courts Constance for Tony and therefore dotes on him. However, upon the arrival of Hastings in the company of Young Marlow, Constance resolves to let Mrs.Hardcastle "suppose that I am in love with her son, and she never once dreams that my affections are fixed on another". So, one can infer that no intentions-to-marry relationship exists between Constance and Tony, except that which Mrs. Hardcastle imposes on them and which they both resist. Constance herself, captures this in ... "I'm sure (Tony) would wish to see me married to anybody but himself."
On the other hand, Kate and Young Marlow are brought together by ties of friendship. Mr. Hardcastle assures Kate thus: ... "I expect the young gentleman I have chosen to be your husband from town this very day". Kate's interest in Marlow is aroused following her father's description of Marlow as "a man of excellent understanding ... very generous ... young and brave ... a bred scholar and very handsome". Interestingly, Kate becomes undeterred even after learning that Marlow is bashful and reserved. Her resolve to "cure him of his timidity" leads her to act as a maid bringing herself down to the level that Marlow is comfortable with. Thus, Mrs. Hardcastle's attempt to force Tony on Constance fails woefully because the motive is selfish and materialistic. Kate and Marlow's relationship endures because it is borne out of attraction. This is shown in the way she "stoops to conquer" Marlow. It is interesting that both relationships are based on pretense — Tony pretending to be in love with Constance and Young Marlow pretending not to be in love, until their circumstances change.
Points to note:
(1) The relationship between Tony and Constance being one of cousins in a contrived courtship bound to fail because of their separate interests and which do not match Mrs. Hardcastle's material interests.
(2) The relationship between Young Marlow and Kate being one based on ties of friendship in an equally contrived courtship which blossoms into love.
(3) Both relationship are based on pretense until the curtain is removed and the parties realize their objectives.