Claiming a Dependent: Brother lived with me for 7 months

I've searched through countless forums on TurboTax and I've read through the IRS website on a qualifying dependent. I am 24 years old and I provided my now 13 year old brother full financial support and living expenses for 2016 for a total of 7 months. He came to live with my the Friday school let out last summer and has been living with me since then. During last years tax year, that puts a total of 7 months into account here. I actually only have documentation for the time he has been enrolled at school, at my address.

I don't have any kind of written permission from my mother or step father giving me the right to claim him on this years tax return. Even tho I rightfully should be allowed too.. but they're being greedy and won't let me claim him. I am scared to file with him on my return anyway. Because my mother will try to claim him anyway. And the last thing I want is my return to be delayed or audited by the IRS for two people claiming the same dependent...

Looking for any kind of advice on this situation. Thank you