Is this theft? Is there anything I can do to get my money back?

I’m 16 and I have a twin sister. My mom and step dad is who we live with. They told us that they wouldn’t buy us a car but they’ll put some money towards it and we’d have to contribute as well. I got a job for the purpose of getting a car. I want to get a fairly decent one to last me a few years. I saved up a total of $6,000 in the past few years. My mom opened up a savings account for me to save for a car and I deposited money in that account frequently. I contributed the $6,000 to the account and my mom has maybe contributed $400. The account is in her name. When she opened this account, it was clear the account was for ME to save up to get a car for MYSELF. I would give her cash and she would deposit it into the account. She put some money in for Christmas, birthday, etc.

Now, my twin sister is pregnant. They say she needs a car. I question why and how they’re going to buy her a car when they said we need to pay for most of it ourselves. My mom said we have the $6,400 for the car. Note, this isn’t a car for both of us. It’s a car for just my sister. I know she needs transportation with a baby, but come on. She has not contributed any money to the account. And now they’re just going to use all of the money I saved on my sister. I’m pissed. I feel like I really screwed myself with the account being in my moms name, but even if it was in my name, I’m under the impression that she could have taken it anyway since I am a minor.