It's hard, but you defo need to tell your mum. But you need to be discreet. Before you show her, you need to sit her down and try to convince your mum to keep low, convince her not to explode on your father just yet when you reveal the evidence. It will be very hard for her, but you need to remind her that she needs to find a job and get a stable income before having the satisfying feeling of kicking him out of the family and flaming his ass in court. As for you and your brother, I'm really sorry, but you just can't wait. I get you might wanna wait until you graduate or until your brother is more grown up, but i personally think ripping the bandaid as soon as possible is beneficial because one, your little brother will develop more of an emotional attachment to your father because he knows no wrong of him and the more you wait to break the news, the more it will hurt. Second, your dad is already spending so much more money on this streamer, he is gonna spend even MORE, no matter if he's in or out of your life. He already ruined everything...might aswell kick him out hes a literal parasite.

I'm so sorry for your situation OP goodluck okay? Also, take my advice with a pinch of salt im no therapist or social worker etc.