PLEASE, PLEASE READ THIS!
I have a younger brother who could be this woman's male twin, so I've learned this the hard way.  PROTECT YOURSELF!
- Set a deadline for her to get out.  The sooner the better.  She's obviously a user and you are just the next sucker she will use as long as you let her.  Do NOT feel sorry for her.  Do NOT believe her excuses.  Ever.
- Don't giver her a dime.
- Don't let her steal from you.  No more phone privileges.  Don't leave cash, checks, credit cards, bank statements or even spare change laying around.  For God's sake (pun intended, you atheist, hehe!) don't co-sign for her!
- Don't leave her home alone in your house.  EVER!
- Be very clear that the dirtbag Mexican boyfriend is NOT allowed in your home.
- Do not let her insult your life, morals, GF, family or anything else.  Do not buy into her guilt and be prepared for her anger when she learns she can't leech off of you.
- Talk about your concerns with your GF now so she knows you are treating her sister like this to protect both of you.  Make sure your GF is on board with the plan and doesn't do anything stupid for her sister behind your back.
- Protect your self-esteem.  Don't buy into the guilt she, and maybe even your GF, will throw your way.  People like her and my brother need to learn they can't leech their whole lives.
I know your OP was just a rant, and you didn't really ask for advice, but I feel compelled to write this.  My brother caused my family a lot of pain and put a lot of pressure on my marriage before I showed him the door for good.  I hope you can avoid such.  Good luck.