Helloooo!!!
I love this idea!! Hoping for the best for your project!!
I wish I could help regarding RPG architect, but I do not have experience programming games (at least not much), but I can give some other suggestions regarding the gameplay, lore, etc.
I loooove story games since I was a kid, and I don't know what you're gonna work around the lore (I mean if you're going to make a horror, a supernatural, a investigation story, etc), but, if that's something I would recommend, is to write the story in advance, or at least, have an ending, a solution for it, just so you have a north to what you will do to the rest of the game. Although I'd find it better to really write the whole thing first, so you have something solid for the whole development (and trust me, in story games, players NOTICE when a story was finished from the beginning, because everything later matches, there is foreshadowing, easter eggs, tips, and other things that scream "EVERYTHING WAS INTENTIONAL").
Another thing are the character. Develop the characters personality and dynamics well, because THIS is what will make the player play to the end. Make characters that feel real, relatable, have the protag have a bond with the sister. You can show that on the intro, flashbacks, small details in the background, the music, dialog, show that on anything, but make the journey feel to the player like something personal, like the one who's playing IS searching for their missing sister.
I have also some tips regarding gameplay, but I don't know if they'll be actually useful. But, try to make interesting gameplay, puzzles, good exploration. Be creative with everything.
And last, but not least, have fun!! Your game does'nt need to be perfect, it just need to be yours <33
I'll be following you and your updates, and I hope to be able to play it in a future not so far!!
God bless, have a nice week and development journey, I'll pray for you <333