Jacob was born before both Tris and Caleb but he would rather have Tris as his sister and not Caleb as his brother. By the time Caleb turned 16 they turn into mortal eneimes after hearing of his betrayl and vowed revenge for what Caleb has done. Jacob seems to get along with both Four and Tris because they both care about him.
Tris Prior: Jacob really loves his sister. During Jacob's 19th birthday Tris gives him a heart necklace containing a photo of her inside if anything were to happen to her.
Four: Jacob's closest friend and one of his greatest allies. Jacob always helps out Four if he was in danger.
Caleb Prior: Jacob and Caleb are mortal enemies after Caleb betrayed their family and Jacob threatned Caleb if he had done anything to harm their sister he would beat him with every inch of his life.
Skylark Hale: Jacob and Skylark are really close with each other and its possible Skylark has a crush on Jacob.