Why Am I So Selfish?

So, there's this thing about me that's been bugging me for a while now. See, I'm the oldest sibling in my family, and we're not really close anymore since I moved out of state for college. My sister, Sarah, is five years younger than me and she still lives with our parents. Recently, my dad mentioned that Sarah is having some financial troubles and asked if I could send her a little money to help her out. He explained that it's just temporary until she can get back on her feet. Honestly, the thought of sending her any money makes me really uncomfortable. Part of it stems from a long-standing resentment towards her for what happened when we were teenagers, she took my boyfriend behind my back and broke my heart. Another part is probably because I’ve worked hard to be financially stable myself and it feels like a step backward to help her out. But, as I sat there trying to decide whether or not to send that money over, I realized something about myself: I'm being pretty selfish. My dad just wants to help my sister, and he's asking me for support because we're family. The truth is, no matter what happened in the past between Sarah and me, it doesn't mean she deserves less from us when she’s struggling. So here's where I’m at: I know I need to do the right thing, but it's not easy admitting that my feelings are getting in the way of something so basic as family support. Maybe this is a chance for personal growth, maybe sending her that money could even help me forgive and move past all those old grudges. Anyway, that’s my confession. I'm hoping sharing this helps me see things more clearly.