The tricky family member is Alexios/Kassandra, the opposite sibling of whoever you're (I had been Kassandra). They have been raised from the FIFA 20 Coins cult and forged into a weapon, and you will square off from them in several of conflicts, and you'll be given the opportunity to kill them (or attempt to kill them) on a number of occasions.
I followed the pleading of my mother and tried to bring Alexios back around again, for him un-brainwashed by convincing him that the cult had been lying to him and using him this whole time. Eventually, this strategy worked and I"defeated" him by having him join my side (and he gave me all his armor, how nice!)
In this storyline you've got the option to buy Fut 20 Coins kill Nikolaos or let him go and act just like you killed him. You are also tasked with finding your mother, Myrrine, and you also discover she's been all over the map doing everything from being a pirate to ruling a little island when you locate her. When there's an opportunity to go against her, I'm not certain when this happens, but if there was a debate about approaches, I usually sided .
The Wolf of Sparta is your father (adoptive father), Nikolaos, that pushed you off a cliff as a kid for killing a village elder (that you did to try and save your baby brother, Alexios), and you discover he ended up adopting another child, Stentor, once you and Alexios were supposed dead, and he is now a Spartan commander.