Responses
You're right but Colica (the Empress ) did that, (send the dress) not by warning ,still,a little it's true, but not the way you think, it's for them to be careful and to also say she doesn't care anymore about covering for the trash emperor when he didnt even defended her family who did everything for him since marriage...
So for now what we know and speculate on-
-Emperor raped ML's mother when both were already married to someone else
-ML born
-House Colica (empress's family) might have killed ML's mum to remove the 'stain' on the emperor.
- Emperor most probably did know what house Colica was planning and tried to use the temple to destroy them
-ML's mum bravely snatched that proof and gave it to MC's father's guild to give to her husband
- there is a good chance that house colica is the one that sent the warning dress most probably belonging to ML's mum to indicate they can kill MC whenever and to stop messing with them (because MC got crown prince's fiancee kicked out to house arrest.