Share your feelings with your siblings and get involved in family discussions. It will make you connected and get rid of negative thoughts.
At times we do feel that way. The best thing is you can share how you feel with them and talk it out. Share what makes you feel like that and see there response. Sometimes the simplest problem can be sorted by having a conversation
I am in agreement with everyone else here. You shouldn't feel abnormal about the situation. It is completely valid for you to feel like your parents favour and elder sibling. And in could be true as well. There have been studies reported which suggest that parents can sometimes have an extra soft corner for one particular child but that by no ways means that you're less loved. Parents are humans too and if you think about it, your elder sibling might be their first born. That could also be a reason for them to have more attachment towards the child. Like everyone else suggested, you must get into a light conversation with them and I'm sure you'll gain clarity about your feelings.
A lot of factors may have to be considered. For instance, what's the age difference between you and your sibling? Also how is your relationship with them? With that said, it's good that you are acknowledging your feelings and wanting to do something about it.
Consider having an honest conversation about your fears. Rather than fighting over issues where you feel so, talk about your underlying concern when there is no conflict taking place. If you are hesitant to have this conversation face to face, write to your parents. Let them know that this is difficult for you to talk about bit important at the same time.
You must be prepared that they may not understand immediately.
It may take them a while before they get you. Don't give up on trying to own what you feel and honestly communicating the same to them.