Hi, I'd like to help you with your problem.
I am sorry to hear that you are in this situation. Sometimes, the people who are closest to us hurt us the most. It sounds like from what you are describing that your family is hurting you by using you and rejecting your requests to be a part of the family outings.
It seems as if the family has long since stopped regarding your feelings as important. There are a few reasons for this, most likely a combination of reasons. One, they are abusive and treat you this way because of that, or two, you somehow became the family target and since no one stops them, the family continues to treat you this way.
The important thing here is for you to decide how you feel about this situation. If you want it to stop, the best option is to refuse to accept their requests for rides, visits, and general impositions. You can tell them in a nice but firm manner than you will no longer be providing services for your siblings and other relatives. Without accusing anyone, explain you feel it is unfair you are treated this way and you would like to be included in the family outings from now on as a equal. This may get you a variety of responses, but it will not be worse than how they are already treating you. They may chose to respond in anger, give you a "cold shoulder" or even laugh. But they will also know that you cannot be used again in the same manner, as long as you stick to what you said.
You can also chose to continue with how things are. However, this will leave you still feeling used and excluded from their activities.
One of the issues I see here is your self esteem. Once you can see yourself just as valuable as your siblings and stop allowing and reacting to their treatment of you, you can start on the road to increased self esteem and a better way of handling people who impose on you inappropriately.
A first step you can take is gaining knowledge and ideas on how to increase your self esteem. One book that might help is The Self-Esteem Workbook by Glenn R. Schiraldi. This can help you get started in finding ways to help yourself deal with your family situation.
If you feel that you need to, consider seeing a therapist for a short time. They can give you support and help you find ways to deal with the family dynamics that occur in your contact with your siblings.
I hope this has helped you,