Sia also had another outburst and the matter of Gloria being not the Hills family daughter also got revealed, and he overheard Sia talking about how the ring that Gloria was brought with was missing and couldn't be found.
It also went missing on the same day that Gloria spoke about she lost something of hers and searched the whole house. What if she came in order to gather all those evidences in the first place?
By acting suspiciously like that, she managed to get on his radar and make him warry, thus leading to the next stage where he hired someone to kill Gloria as well as Faith. And that's where things get interesting.
"Mother, if Gloria knew about everything from the start, then it will also make sense that Xander knew. If those two were together in the sense that I find repulsive, then when the call came for her to meet with Sia to talk it out and at a place that Sia suggested, they should have been wary from the start."
