Hi, I'd like to help you with your question.
As I was reading your post, it occurred to me that in your life you do a lot for others, but not much for yourself. I was also stuck by the fact that you seem to view yourself as lower than others and therefore worthy of less. I believe that this may be the root of why you feel the way you do.
First, being the caregiver of your parents is enormously stressful. That is known to cause people to feel overwhelmed and depressed. Given that your siblings are not helping you (I would assume in any capacity), then I would be surprised if you were not feeling depressed.
Also, the fact that you are aware that your friends contact you last, when all other options are exhausted for companionship is not a good thing. Since I do not know the reason for this, I am going to assume that they treat you badly because either they are just that way as people, or that you allow the behavior. Either way, your acceptance of the behavior should change if you want to feel any better.
One important step you need to take is seeking out a counselor. At this point, it would be a very good start and a way to help you begin to restructure you life and your interactions with others.
Some of the things to work on with your therapist are gaining self esteem, putting limits on your relationships and the behavior you will accept and asserting yourself in situations like caring for your parents. At this point, no matter how busy your siblings are, they need to contribute at least some of the time to your parents' care.
There are also some books I recommend for you to get started. One is called Healing Your Emotional Self: A Powerful Program to Help You Raise Your Self-Esteem, Quiet Your Inner Critic, and Overcome Your Shame by Beverly Engel and another is The Courage to Be Yourself: A Woman's Guide to Emotional Strength and Self-Esteem by Sue Patton Thoele. These are both available on Amazon.com or your local library might have them available.
Keep in mind, you deserve just as much as anyone else. You mentioned being a Christian (I am one as well) and although God wants us to be kind to others there is no where in the Bible that says we have to accept being abused. You do not have to accept this either. God made you just as well as everyone else. Remember that as you journey to help yourself get better.
I hope this has helped you,