With regards to if sperm donor knows that his wife is a foreign princess, I can see there being a chance but a very small one. The ruing family seems to know and should more then likely feel they need to tell him, even if it was being kept a secret , to ensure he treated her as her station demands, and he does not do anything stupid that risks a war with a superior nation. like he has been doing for apparently years.
That a member of the hosts family came out to escort them seems to indicate that that family at least does not want to insult her, as if she was just the disfavored wife and shunned son that the way they are treated should indicate and have spread in rumors why bother sending anyone anyway important, as a higher ranking servant would do.
@cor3zone : Look at the rumors about the MC at the party, saying he has a bad temperament, this feels like an attempt to get them to cause a scene to justify the treatment and neglect, and possibly even disowning the MC.
While the father might have not caused the issue, that he :
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[*] Has not done anything to address the rumors about his son
[*] Did not ask what happened when they were missing and then send a messenger with an apology for the "mix up"
[*] Left for another party without asking them to accompany him, or even bother telling them he was leaving (or assumed they left without telling him they were leaving)
[*] The children limit was apparently written on the invitation he almost definitively read
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Makes him 100% complicit. and shows how little he thinks of them. Also keep in mind he is the one who suggested they separate, which kind of makes no sense as the family should enter together, and in proper order. the separation is a hug red flag he knew something was up. Hell, realistically all 5 should have arrived in the same carriage.