Mrs. Hardcastle, as the aunt of Miss Neville, has control of her possessions, including her jewels that are obviously very valuable. Note how Constance refers to these jewels when Hastings tries to convince her to elope immediately with him:
I have often told you, that though ready to obey you, I yet should leave my little fortune behind with reluctance. The greatest part of it was left me by my uncle, the India Director, and chiefly consists in jewels.
Mrs. Hardcastle knows the value of these jewels, and this is why she is so keen to arrange a match between her son Toby and Constance, so that the wealth will be kept in her family rather than go to whoever Constance marries. Of course, as Mrs. Hardcastle keeps the jewels locked up for safety, were Constance to elope before gaining possession of them, she would lose them and Mrs. Hardcastle would have her hearts desire, which is to retain control of these jewels for herself. Mrs. Hardcastle is a character who is clearly shown to care more about the jewels than she does about the happiness of those around her.