Non-mahrams

Are the children of one's adopted father's second wife mahrams?

Q: I'm adopted by my parents. My adopted mother then breastfed me so they became my foster parents. In 2021, my mother (foster mother) passed away and my father got married to another woman. She already has three children. Are they mahraam to me? Should I have to make purdah from them? Is it permissible to marry them at the same time?

Having meals with non-mahrams

Q: Is it permissible for me to sit and eat with my girl cousins while her father, mother, brother and my mother are also seated there.

Also, can I say جَزَاكَ اللَّهُ خَيْرًا and speak about general things like Madrasah with her mother, who is my mother's brother's wife while they are wearing abaya and scarf but part of their hair or neck is exposed?