The official CV of Christopher O'Neill talked only a little about his family except mentioning his parents and five half sisters. So let's take a look at Madeleine's future in-law family.
Eva Maria O'Neill, née Walter was born in 1940 in Austria. She was married and divorced four times. From her first marriage to Walter Schoeller, who worked as Indonesian consul in Munich, she has daughter Tatjana Schoeller. Tatjana is married to Henry d'Abo son of Robert d'Abo and Lady Ursula Manners, together they have three children Anoushka Maria Violet (*1991), Celina Maria Isabel (*1996) & Jasper Henry Erland (*2001).
Daughter Natascha Loeb comes from her second marriage to Hermann Loeb. Natascha is married to Graf Enst von Abensperg und Traun and together they have one daughter Milana Maria Tatjana (*1996).
Her third marriage was to Christopher's father, Paul Cesario O'Neill. Her last known marriage was to James Elliot Steward.
Paul Cesario O'Neill was born in 1926, as son of Paul O'Neill & Josephine Cesario, he passed away in 2004. Before his marriage to Eva Walter, he was married to Hildegard O'Neill with whom he had three daughters Annalisa, Stephanie (Stefanie according to her mother's orbituary) and Karen. One of them is mother to Oliver. Unlike with Christopher's maternal sisters there is very little known about his paternal ones.
Abensperg und Traun
Orbituary of Hildegard O'Neill