Family Drama

Background: My addict sisters middle daughters name is Kc. She turns 16 tomorrow. When she was 14 she was in this relationship with this 16 year old boy. They were together a year and some, had alot of ups and downs (they are teenagers!) and she actually suffered a miscarriage at the end of the relationship. Do I think they should of been having sex? No.  Did I think the age mattered at that age? Yes.  Was anything I thought going to stop the relationship? No.  So anyways, they break up right after she turns 15.  She dates one other guy in 10 months for a short while.  A few days ago she broke up with that one guy and announced she was going back with C, the previous boyfriend.
 
Now, most of our family doesn't care for C as after the misscarriage he wasn't a very nice person.  His family kicked him out when they found out he was with Kc, and when they found out she was pregnant it all went to hell.  In the end he chose to go back home, and that meant not being with Kc.  Now that they are back together she is asking the family to give him another chance.  To let him prove that he's changed.
Her mom has flown off the handle, called her a...pardon my language...stunned c*nt.  Because she's so perfect you know.  My mother, Kc's grandma who she lives with, is letting Kc's ex stay at her house until he finds a place to live and Kc won't go home, as her ex has threatened C's life now.  My little sister, her other aunt, had a yell out with Kc, but I've since talked to her and she has apologized because she get's where I am with this now.
I just...dont...get....it.  She's 15, it's not a life ending decision who she dates.  Sure, he treated her poorly, and sure, he may do it again.  But that' not our decision to make.  And I stand by that, causing alot of my family to be angry with me, big sister ecspecially.  Because they think I've magically forgotten what C did to her.  No, I haven't, but getting my panties in a bunch over something so minor in the grande scheme of things is silly.
The road to self discovery is a long and winding one, what you use for stepping stones is soley up to you, letting someone else chose your steps means you are letting someone else shape your life.