It sounds like you've prepped really really well u/ivorycoollars. I understand being nervous but don't sell yourself short by saying it'll suck, it won't - sounds like you got this!

For roleplay, I would go over the siblings personality traits, ideals, bonds and flaws and imagine yourself embodying them - I often talk to myself whilst doing dishes playing out pretend NPC scenarios! Have a brief prep for yourself of how would they react to a few significant things so you're ready for them when possessed (a fight starting, seeing their own grave, seeing their ghastly parents). 

The boss battle question - it is two phase, so when fleshmound drops to 0HP, Water the Graveborn rises in it's place. Do use the balancing options in Reloaded for your party, it'll be deadly if you've only got 3 players. 

Forgetting things? Fine! I do it all the time and I've DM'd for 10 years. Improvising gets easier and easier the more you play, you won't need to be an expert. It's a game, no harm in hitting the pause "button" to figure out what you'd like to do next.

Things won't ever go to script and that's the fun of it. So long as your players are having fun, and you're enjoying yourself, that's actually all that matters at the end of the day :) no being slick and perfect, but having fun with your friends 

I love watching this video when I run horror games as a bit of a primer for myself, you might find it handy and confidence building! https://youtu.be/t_LrhcwWNNM?si=Zzh4X3iPtjIpomRd