In the last episode we saw, when Sid's mom said Riddhima that she's Armaan's wife she shouted, "Armaan mera pati nahin hain!"
Riddhima also said angrily to her, "Baas! Main aur sun nahin saakti!" Though this is an misunderstanding, but Riddhima should not talk to her like this. Many of you will now say that, that time she was not in her mind. Okay, okay, I'm agreeing. But later she should understand this; she misbehaved with her mother mother-in-law and should apologize to her. But later she didn't do that. She didn't apologize to her mother-in-law!
In the beginning of the show our Ridz was not like that. That time she used to apologize to everyone for every little mistake.
And the most important thing, Riddhima isn't realizing that Sid's mom is her mother-in-law! That's why Riddhima addresses her as 'Aunty' in her behind also and addresses her to Sid 'tumhaari mom'. We know that Sid's mom doesn't know that they're married. So that Ridz can address her like this. But why in her behind Riddhima never addressed her as just 'mom' or 'mommyji'?