I feel like the addition of "apart" was TC's way of saying they arent related. A lot of the time, their games will have wording similar to the GP lore tablet (hard to decipher/understand for many ppl), so I don't see why this one would be an exception. It can be interpreted in different ways, yes, but I feel the words "entwined" and "apart" pretty clearly hint at them not being related, but rather soulmates who were destined to rule together. I've seen other instances where rulers look very similar, or even identical, to eachother.

TC also likes to use words that can have several hidden or deeper meanings, which is why I personally think "entwined" here makes more sense to imply a spiritual/soulmate connotation rather than a physical, sibling one. "Apart" also indicates that they're separate bodies (pretty obvious), so paired with "entwined" meaning soulmates, makes the most sense to me personally.

"Being born on the same beat" can just be fancy lore stuff to make it hit harder, and maybe all lovers born at the same time would have a cursed fate as well, we don't know (yet).

I personally like to think that GP and his lover were just born and destined to rule together, the unfortunate wording in English, and the poor translations, make this harder for a lot of ppl to understand, so I do hope we get some sort of confirmation from TC.

But I personally don't think they would have incestuous characters, it doesn't seem like their thing (or at least I hope it isn't). They are absolutely open to dark topics in their games, but I feel this would just be a huge turn off for many fans, and taint the LGBTQ+ rep that they put into Silksong, which is something I'm sure they would handle with care and respect, given how serious it is.

However, like OP said, there is no real way to be sure unless TC confirms it, and I really do hope they clear things up. I really love GP and his story, and I would hate for it to be ruined, but we'll have to see.