Giving to Family

My older sister called me (at 0645 on a Saturday) the other day and asked me for $500. She is not on drugs and is a married mother to two beautiful girls aged 15 and 11. She has been calling everyone in the family for money for years and years. I think the last time she asked me I told her not to ask me again. Not that I don't love her, but she is bad with money. If she had $1 or $1,000,000 she would spend it all. So, I call my younger sister and asked her what was going on and asked her what she thought... She said, don't give it to her. She said that two out of the last four weeks she had not worked and one of those weeks was for a vacation. I spoke with my older sister and told her I could not give her the money. I don't know jack about my sister financials, but I know they have never been good. They do own their home, which my dad gave them the down payment for... that is probably tripled mortgaged by now...

I don't have any children, nor did I get knocked up at age 22 without medical insurance or a job. I have worked 50+ hours a week for the past 15 years and so has my wife. I have a ton of money in the bank.

Why do I feel like a piece of shit?