Mother Of Kings: Families And Alliances

The lendman Ulf/Kveldulf
sons: Thorolf; Grim/Skallagrim

Skallagrim
sons: Thorolf; Egil
daughters: Saeunn; Thorunn
foster brother: Thorir Hroaldsson
foster daughter: Aasgerd

The hersir Thorir Hroaldsson
sister and ward: Thora Orfrey-Sleeve
son: Arinbjorn
foster son: Eirik Haraldsson
foster brother: Skallagrim

Bjorn Brynjolfsson
father: the hersir Brynjolf
wife: Thora Orfrey-Sleeve
daughters: Aasgerd; another by a second wife

Thorolf Skallagrimsson
brother: Egil
wife: Aasgerd
daughter: Thordis

Egil Skallagrimsson
brother: Thorolf
wife: Aasgerd (after Thorolf's death)

Ozur Dapplebeard
wife: Kraka
leman: Seija
sons: Aalf; Eyvind
daughter: Gunnhild
son-in-law: Eirik Haraldsson

Onund/Berg-Onund
father: Thorgeir Thornfoot
wife: Bjorn Brynjolfsson's second daughter

Arinbjorn is Bjorn Brynjolfsson's first wife's brother's son, thus Bjorn's nephew-in-law.
Onund is Bjorn's second wife's daughter's husband, thus Bjorn's son-in-law.
Onund says:

"'...now Arinbjorn and I have the same ties to Bjorn Brynjolfsson.'" (Mother Of Kings, p. 145)

I wondered why he said that and have had to write this post to find out.