Engaged sims are suddenly siblings

I have two sims who are engaged and have two kids already but after logging off and bacck on the next day they are suddenly considered siblings with an "unknown" common parent. I am playing with some mods but none that interfere with relationships and no cheats or MCCC can fix this issue. Unfortunately, I noticed this bug way too late and I'm unable to load an old enough save to continue playing with my current household with my sims remaining only as romantic partners and (quite honestly) a normal family.

Also the royal envoy won't stop comming to my household - I don't have the royalty dlc and not planning on buying it - it's getting pretty annoying as she just walks into my sims' house and keeps talking to them.