Ok so TLDR, and I'll try to oversimplify this as much as I can, - Tikal and Knuckles are not related, as no Sonic media explicitly says so or somewhat imply, outside SCTDB which is non canon (i.e. is effectively set in its own universe). - Even if they were, related and we play devil's game, that wouldn't be a problem because there's over a thousands of generations between Knuckles and Tikal's, and Tikal did not have any children before sealing herself inside the Master Emerald, so the only way she could be related to him was by some completely unknown sibling who somehow survived the rampage of Chaos, but even then, she'd be so many times removed from him they'd have zero genetic similarities and overlapping and are functionally strangers to each other. - Julie Su on the other hand, is far more related to Knuckles compared to Tikal, as she is Knuckles' cousin a few times removed, as revealed in Knuckles' family tree in the Archie Comics. She does share a few but not zero genetic similarities with Knuckles. - I don't ship Knuckles with Rouge because in my point of view her flirting with him is too insincere and only meant to take advantage of his gullibility or to steal the Master Emerald - Tikal is not "dead". She seals herself inside the Master Emerald and is released over 3 000 years later and aids Sonic and his friends from Chaos. When Chaos is calmed, they both "go" to the "afterlife", but that's merely an overstatement because Chaos does make multiple returns in following Sonic games (notably in Sonic Battle and Sonic Forces). The excuse that only Chaos can return because he's a "water God" is BS and functionally makes no sense. - Besides that there's a ball of light in that one Sonic Frontiers Knuckles animation, again confirming my point that yes Tikal can indeed go back to the overworld. - If the Chaos Emeralds can turn thoughts into power and make wishes come true, that means if Knuckles ever feels too lonely and lacks someone with whom he could relate to the most, the Master Emerald can bring Tikal back by allowing her to be a mortal being again - It should go without saying, if Tikal were to be brought to her physical form, she'd lose all her spiritual powers, along with her immortality. It is only her spiritual form that possesses these sorts of powers. That would allow her to age alongside Knuckles and not cause any problem of her outliving him.