Is my mum being unreasonable or am i just being childish?

I agree with you! Your mom is probably just being blindsided by her boyfriend, so she doesn't have time for you guys. You know parents always say that we are selfish and lazy, but in your circumstance it is the other way around! Tell your mom how you feel and if she still gives you grief, then I'm sorry your just going to have to obey because after all, she's the adult(even though your acting like one and she is like a teenager!) I wish you the best of luck!