So he's her uncle. Both girls are step daughters but they're still technically related now and MC doesn't know this.
Also seems possible that the fathers death was not simply a murder but something he chose? A sacrifice? MC's dad wishes for his daughter's forgiveness. This girl tells him that he should not expect that forgiveness, as she also can not forgive "him" for "giving up his life." That seems to imply that the "him" she's referring to is her father or someone similar. This girl seems to reject this apparently self sacrificial act that MCs father is apparently involved in, by showing him how a similar situation (hers) ended up.
The only thing I'm unsure of is whether this girl really is referring to her step father or her bio father. I'm not sure if her orphanage origin was true or not. The sacrifice could have been the guy who we see murdered, or if it could be someone else? Still need more info. It's fun though.