Mycen never tried to say Alm and Celica were siblings. I can see the point, but it's not really incest. I see Alm and Celica as more like Simba and Nala, where it's like "holy hell you grew up hot" Alm was totally holding back a boner when Celica tackled him lol. The fight? Awkward sexual tension I guess.
If there was anything between Celica and Saber it was one sided from Celica's end. Saber's a dashing rogue, but he's also old enough to be her father and if there's laws against pedophilia then yeah, and plus I think he's attracted to women his own age.
The fact that both Saber and Lima drink too much is a good point too. Celica saw how much of a douchebag Lima is so she doesn't want that in a partner. Lima may have even been downright abusive to Liprica and his other wives when he was drunk. No girl wants that. Drunk Saber might get aggressive.
Please nobody bring up Corrin being able to bed his/her adopted siblings because Fates writing is terribad and does not count.