When Dev reacted to Sona's talk angrily it was quite natural. He wanted her to stop. A lot of things from his past just came back to his mind with the talk and its but natural for him to react the way he did. An instant sorry would have meant he lacked the emotions to what he just said, which he didn't, it was deep rooted pain, pain accumulated over several years and I am glad he didn't say sorry right there. Again its natural. He takes care of Sona when she needs it. He does a lot of things for her, given the fact he has recently learnt how to live a happy emotion-filled life.
To say he noticed her saying Maa might have been Dev's way to cheer up Sona. He doesn't know any better way n secondly they were talking about Ishwari so its in flow for him to say that.
Sona understands him, she knows he didn't mean to hurt her. It was her attempt to calm him down when she said sorry. She knew she had hurt him by forcing him to remember the painful past memories. A young child going through all that pain is truly unimaginable for anyone no matter how much the person cares for you.
Lastly for Dev to know about her family, I guess that time has not come yet. He n her are yet to reveal about the relationship to the family's. Perhaps we would see more of Bose family after confession in front of Ishwari. Dixit house is practically sona's second home away from home as she spends most of her time in the house. So getting involved with People is expected n she is more social than Dev for sure.