The prospective bride and her relatives mentioned in Richard's posting do not appear in the articles on the Marquess of Waterford's family in Burke's and Debrett's peerages. However, it appears that her great-grandfather was Charles George de la Poer Beresford (1861 - 11 VII 1937), who can be found in the pedigree in Burke's Peerage, although no marriage or descendants are shown for him there.
Charles George de la Poer Beresford was a 3 x greats-grandson of the 1st Earl of Tyrone, who was the father of the 1st Marquess of Waterford. He was married in 1914 (registered Q2 - Chelsea) to Ethel Annie RAYNER (10 XII 1892 - 1975); per Free BMD, it would appear that they had seven children, the sixth child and only son being John Charles de la Poer BERESFORD (23 VI 1923 - 1977; see https://www.chiles-lamanfh.com/obituaries/evelyn-beresford-thorne/1407/ for an obituary of his third sister).
John Charles de la Poer Beresford married (Mavis) June Mary GUILFORD-GOODE (died 17 XII 2019) in South Africa on 31 December 1953 (see https://www.1820settlers.com/genealogy/settlerbrowsemarrs.php?name=GUILFORD-GOODE&first=G). I can only find the births of two of their children on Free DMD: Julian N M C de la P BERESFORD (registered Q2 1960 - Surrey South Eastern) and Simon J A de la P BERESFORD (registered Q1 1963 - Surrey South Eastern); as Jilly appears to have been their eldest child, it may be that she was born abroad.