Vampiric Siblings

[255 for Demons!]
This is the usual Tensura world except for one thing, a certain demon lord has an unknown sibling equaling to a demon lord seed's power and has the amount of potential Rimuru has. To add, this story is my very first so please don't be mad if it's bad or doesn't make sense. If you have any suggestions relating to anime fanfictions, then please comment. However, some anime is unreachable to me and I prefer ones like TTIGRAAS and such. Also, the ship is still undecided, so please send me messages or comments giving me suggestions for ships. Whichever ship is wanted the most will be it.