While it while since I last saw mlp show but I get the feeling that Sombra might be Cadence’s biological father.

Doesn't make any sense. Sombra was born over a thousand years ago and was banished when Cadance was born.

IIRC, Cadance was a pegasus adopted by earth pony parents. Where she came from, is open to interpretation. The Crystal Empire? Krypton? Nopony knows!

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Sometimes it's a fun idea to play around with

Not directly his daughter, but could be his direct descendant.

Yes, he is, or related in some way. How else could she claim dominion over the Crystal Empire?