i am sorry i know how that goes with my two younger brothers

it is not a matter of love....she loves her children...both of them....just different relationship with you.....and stop comparing yourself to someone else....that is a bad thing for most of us to do....we do it and it just pulls us downwards...

My mom and older sister have a strong relationship. They always have. It's just sometimes my mom doesn't realize she needs one with her other daughters too.

There is a term "golden child" that describes when a parent love one child more or favor one child more. Now your mother may or may not truly favor the sister but I mention this term because it does happen in some families. There is another term called narcissistic family dynamic where one child is the golden child and another is the black sheep or scapegoat, the one considered less than or bad. I am going to suggest you really start to observe and listen to your mom in her interactions with you. It might be that you need so much validation from her that you talk longer than you think you are talking and when another person or sister walks in, then the natural bend in the dynamics is mom engages with the sister. Might not mean she favors sister. But if you observe yourself and mom, you will start to notice who does what. Instead of viewing this from your emotions, step out of your emotions and observe and listen. Pay attention. If you decide after observation and listening that you are the scapegoat or unfavored child, this is something you have to acknowledge and accept and heal from. After observing and listening to behavioral patterns including your own, you can attempt to speak to your mom about it. Or you can change how you act and react (this is probably the most healthy route). We can't change others but we can change ourselves. If your mom has flaws, they are not about you as the cause, even if she says that or her preference of sister makes you feel that. If our parents lack the capacity to love us fully, it isn't our fault. Their lack of love doesn't mean we are actually flawed. Some parents, well some people, lack the capacity to give fully to another and are instead self absorbed. If it is you that is attention seeking and long winded, you can change that so that mom's attention hits the mark. Best wishes to you.

I think my twin sister and I are the Scapegoats while my older sister is the Golden Child...
I don't really want to talk to my mom about it, though, because whenever I try to talk to her about what I'm feeling, she likes to make it sound like she knows better than me and that I am just "adding on symptoms". She does it all the time when I tell her about my Anxiety and Depression. I'm starting to become more Introverted and afraid to even talk to her about what I've been learning and stuff I'm excited about.
Oh, wow. I didn't know we had so many of these symptoms. I didn't even tell everything.