This is a really tough situation, but you are ultimately enabling them. I was in a very similar situation with a relative (not my parents). I was constantly being tapped for rent money, money for prescription meds, money for groceries, I was once even stiffed with an $80 restaurant bill when this relative was supposed to be picking up most of it. However, this person often had money for DVDs, eating out, and so on.
You have to just STOP loaning them money. They need to learn to live within their budget. It's hard, and it's going to cause friction in the family (at least temporarily), but the situation will only change if you change your behavior.
You can't control if your other siblings give them money, but you can pull yourself out of the situation. It sounds like there is a lot of unhealthy stuff going on, with the lying, too. I am not being judgmental--my family has its own issues, too--but you have to create healthy boundaries for yourself.
Just keep telling your mom "no" when she asks for money. You might surprised at how quickly she realizes you are serious, so long as you consistently say "no". My relative caught on pretty quickly and moved on to mooching other people.