For your amusement, here are some examples of weirdly inter-related noble families in the thirteenth and fourteenth centuries.
The de Clare siblings' half-nephew marries their half-sister
Gilbert 'the Red' de Clare, earl of Gloucester (1243-1295), and his second wife Joan of Acre (1272-1307) had four children, Gilbert, Eleanor, Margaret and Elizabeth, born between 1291 and 1295 and the nephew and nieces of Edward II. Gilbert the Red also had two daughters with his first wife Alice de Lusignan, Isabella (born 1262) and Joan (born c. 1264). Joan de Clare was the mother of Duncan MacDuff, earl of Fife, who was born in late 1289: he was the half-nephew of Gilbert, Eleanor, Margaret and Elizabeth de Clare, albeit some years their senior.
Sometime after 1307, Duncan MacDuff married Mary de Monthermer, who was born in 1297, and their only child Isabella was born in about 1320. And who was Mary? Daughter of Joan of Acre and her second husband Ralph de Monthermer, and thus the younger half-sister of Gilbert, Eleanor, Margaret and Elizabeth de Clare. And so, the half-nephew of the de Clare siblings married their half-sister; the grandson of their father's first marriage married the daughter of their mother's second marriage.
The earl of Derby's daughter becomes the stepmother of her own stepmother
William Ferrers, earl of Derby (1193-1254), married Sybil Marshal, one of the five daughters of the great William Marshal, earl of Pembroke (d. 1219) and Isabella de Clare. William and Sybil had seven daughters. After Sybil's death, William married his second wife Margaret de Quincy, daughter of Roger de Quincy, earl of Winchester. With her, he had his heir Robert, earl of Derby, born in about 1239, a younger son and three more daughters.
Eleanor Ferrers, sixth or seventh of the seven daughters of William and his first wife Sybil Marshal, married Roger de Quincy, earl of Winchester - the same Roger whose daughter Margaret married Eleanor's father William Ferrers. Eleanor thus became the stepmother of her stepmother. For William Ferrers' five younger children, this meant that their half-sister married their grandfather. It's probably just as well that Eleanor Ferrers and Roger de Quincy had no children, or the universe would have exploded.
Roger Mortimer's grandmother marries her step-grandmother's son (thanks to Ann Marie Thomas on Twitter for this one)
The paternal grandmother of Roger Mortimer, first earl of March (1287-1330) was Maud de Braose (c. 1224/28-1301). Maud's mother Eva was a daughter of William Marshal, so that Maud was a first cousin of the seven Ferrers sisters, above; her father William was hanged by Llywelyn ap Iorwerth, prince of Gwynedd, in 1230 after being found in the bedroom of Llywelyn's wife Joan (King John's illegitimate daughter). William de Braose was the son of Reginald de Braose (d. 1228) and his first wife Grecia Briwere. Reginald, the grandfather of Maud de Braose, married secondly Gwladus Ddu (d. 1251), daughter of Llywelyn ap Iorwerth who hanged Reginald's son William.
After Reginald de Braose's death, Gwladus Ddu married secondly Ralph Mortimer, and was the mother of Roger Mortimer (d. 1282) who married Maud de Braose and was the grandfather of Roger Mortimer, first earl of March. Gwladus Ddu was thus both Maud's mother-in-law and step-grandmother. It is uncertain whether Gwladus was the daughter of King John's illegitimate daughter Joan or of Llywelyn's mistress Tangwystl, but if the former, it means William de Braose had an affair with the mother of his stepmother, and if the latter, that he had an affair with the stepmother of his stepmother. For Maud, it meant that Llywelyn, the man who had hanged her father, was her grandfather-in-law.
Isabella of France's grandmother is her husband Edward II's aunt by marriage
Blanche of Artois (c. 1248-1302) was the niece of Louis IX of France, and married firstly Enrique I, king of Navarre, with whom she had a daughter Jeanne or Joan, queen of Navarre in her own right, who married Philip IV of France. Joan of Navarre and Philip IV were the parents of Isabella of France, Blanche's granddaughter. Blanche married secondly Edmund of Lancaster, Edward I's younger brother, and was thus the aunt by marriage of Isabella's husband Edward II.