I think you have quite a lot of stress on your hands. I can certainly understand the annoyance of having two siblings. I am the middle child in my family with an older and younger sister. They do annoy me, and we do fight, but at the end of the day it's always about small insignificant things. The main point here is that whatever they say or tease you about, you are their sibling and they do love you. They'd hate to see you sad or hurt, whatever you think, or however angry you may be with each other. I think that it's best for you to talk to someone, be it a friend, or a counselor. While it's better to see someone professional, I appreciate that not everyone wants to or can, and in that case even a close friend or someone else you can trust is fine. You need to get things off your chest, it seems to bottle up all these emotions, and sometimes these all get released together which is why you lash out. I know, I've had similar problems. That is not the way forward and only increases your stress and anxiety levels.
It's good that you're talking to your best friend, and while this is helpful, it might not be the best to unload everything onto her. It can put her in an awkward position if she doesn't know how to respond to something. Here a professional counselor is good, because they can talk to you, and give you advice specific to what you tell them. Even so, just talk through it with your friend, and as you bottle things up, a slow release is good. If your friend can do it, just talk to her a few times a week about what's going on and what annoys you, to let it out and not keep it all in.
Problems like these don't just go away. You have to work with it, if you continue to just keep things inside all the time and never release anything it can get worse and lead to something disastrous. I'm sure neither your mum or your siblings hate you. While it certainly might feel that way, there is always sibling rivalry in families and feuds between siblings. This is entirely normal, it just might be more pronounced in one family than others.
No family is perfect, no person is perfect. Don't worry, no-one expects you to be perfect. Your parents sometimes do things that may make you feel like they want you to be perfect. They don't. They just want you to try your best at everything to make sure you have the best chance in life. Parents put pressure on you because they care about you and they want you to do well. Siblings fight because they're jealous of each other, because one gets more attention than the other, and many other reasons. But don't think that any of them don't love you. Because no matter what they say, deep down you are their sister, and they do love you.
Just keep being strong. I know it seems like forever, and that it will not go away, but talking to someone on a regular or semi-regular basis can help release tension that you feel. Just rant away to someone, but don't keep those feelings locked up inside. Try to be more happy throughout your day, make jokes, talk to your friends and enjoy the time of your youth. Play games, watch TV or invite a friend around to do something to take your mind off things. It can definitely help relax you and calm you down so you don't lash out.