Elsa and Anna's parents were on they're way to Rapunzel and Flynn Rider's wedding and Rapunzel is Elsa and Anna's cousin.
Ehhh........are Tangled and Frozen even from the same time period? This entire theory is based on one cameo appearance that Rapunzel and Flynn have in Frozen even though Elsa and Anna NEVER interact with her AND because Rapunzel looks low key similar to Elsa and Anna (with obvious differences with her hair, eyes, dress, and power), which considering Disney's style of animation......it's probably just because of that.
However.......I kinda like both of these theories. However, I don't believe they are canon. Frozen 2 probably messed this theory up anyway. And besides if Rapunzel was Elsa and Anna's cousin......then why the hell does she never interact with them what's so ever? Why would they not be at Rapunzel's wedding? Why would they absolutely NEVER show up again? Why would Rapunzel not care what's so ever about Elsa and Anna's well beings.
It's not a bad theory and it works well in FanFiction but that's all it is.