I hope this isn't offensive, it is not meant to be. Other TG-guys have told me on posts here that it is easier for them to transition as 'girls' can present anyway they want in society and be accepted. While a MtF 6' 5" line backer in a mini and heels will probably be safe

but would not prevent being looked at as 'odd'.
Why don't you, for now, just say you do not like the feminine look, that you prefer to present how you feel and not some 'pretty lady'. Jeans, T-s etc are your kick not dresses and skirts. You don't like the feel of make up and really don't think it enhances your image. I know it may sound like playing the female pitch with your mother but sometimes 'white-lies' are needed. If you have longish hair put it in a low pony tail as other guys do. Don't put it into the perky, girly one (

wish I could), there are lots of ways to slowly tone the image down so that you end up presenting rather gender neutral. Also VERY important, be happy. If your mum and her BF see you as a happy content person they will let things go. If you present as sullen and upset they will start thinking the obvious stuff eg: Oh daughter is unhappy; she must not be feeling attractive, she can't find a boyfriend, let's get her some nice feminine clothes to help her, we can even book her in for a makeover; she'll be happy then. DUHH, yep folks that is how parents think, You are to your Mum still her daughter, she regards you as female, she, being female, is responding in the only way she is programmed. I will help my daughter through this difficult time, after all WE know what teenagers are like.
The truth is illogical to her, so my suggestion is to take it slowly and plan. Accepting her non-acceptance may sound down right dumb, but in your situation it may make sense.
Hugs
Cindy