My mom's on drugs again, how to tell Dad? HELP

A

Story time! My dad is no stranger to drugs, he was an alcoholic for a big chunk of his life which led him to experiment with quite a bit of hard drugs. My dad has been sober for 14 years, my mother cheated on him when I was about 14 and they split up. A little while after they split up my mom started seeing this guy ( not the guy she cheated on my dad with) the dude seemed eehh alright at first. I was skeptical cause like fuck this dude he's not my dad yknow? They moved pretty quickly, he moved in (we lived with my grandma at the time) within the first 3 or 4 months of them being together. I didn't hate the guy cause he would give me all the weed my little heart disired 🤷 not too long after he moved in my mom started acting a bit weird, my mom is already quite the weirdo, but she was sketchy as hell. She would kinda try to stay away from everyone, she'd be doing a bunch of odd jobs, staying up until like 5 am, rambling off talking all kinds of nonsense, just not in her right mind. One night when my friend and I were looking around the garage for weed we found a bag of white shit, showed it to my older sister and she had told us it looked like meth. From then on we were all on the hunt for more evidence, we then found pipes laying around the house and my sister told my grandma, they had told my mom she needs to stop her shit or move out. My mom then moved out to a friend's house and I still lived with my grandma for a bit until I moved in with my dad once he got an apartment. I was told not to tell him because they were affraid my dad would take my younger brother away from them and that they would handle it and talk to him about it and that I shouldn't worry about it cause I was, now at the time, 15. That always pissed me off but I honestly had no idea what to do, so I dropped it never talked to him about it. My mom eventually moved in with her boyfriend on his friends property and made a house out of a barn!! (Shitty shitty little shack they lived in) my dad would drop my brother and I off at her place in the morning cause he had work and we needed to get to school, after a while I told him not to take us there anymore cause it was sketchy and I told him the people around there were sketchy and probably tweekers (they definitely were) so he'd take us to my grams for the next 3 years, except some days my brother would want to go to Mom's or my mom would just take him over there, I would pretty much just be where ever he was cause I wanted to protect him. My dad would like kinda hint to the fact that my mom and the people she was around were twaks, and my dad isn't stupid, if you seen the place my mom lived you almost instantly knew so I just always figured he had an idea of whats going on and I was pretty positive my grandma talked to him. My mom would off and on be on the drug within those years, you could tell when she was on it and when she was off. Well as of the last 2 years she's been mostly off it cause she hasn't been with her boyfriend ( here and there she was) but again it's easy to tell when she's on and off. I live with my sister now and my brother is either at my dad's or Grandma's and my mom visits him at my grandma's. But recently she's been back together with her boyfriend and acting sketchy so everyone in the family has been skeptical. the other day she came over and we found a crack pipe in her wallet. My sister and I talked to our aunts and we think the best option for her is to put her into a rehab, she has nowhere to go right now except for places where people are tweeking 🤷 we're going to have an intervention with her when my aunt comes into town next week and that's when we're hopefully going to put her in a rehab. I obviously have to talk to my dad about it cause that's the mother of his kids but I just don't know how or when I should do it? I know he's going to be upset probably and it's really hard for me to get a ride to his house since my car took a shit. I don't know what I'm going to say and I'm kinda pissed that all of a sudden it's all on me now🤷 I'm doing it alone and my sister isn't keeping me updated on what the fuck is going on. My boyfriend and I are supposed to be spending the weekend with him and my brother this weekend and then I'm assuming we're talking to my mom that following week. I'm affraid if I tell him before this weekend it'll ruin it, if I tell him during it'll ruin it, and if I tell him after he'll be extremely pissed cause I waited so long to tell him. I just really don't know what to do and no one is helping me figure it out. Any advice would be helpful please no negative comments I'm already in a pretty low spot rn among other things.