Kashibai
Kashibai was the wife of Peshwa Bajirao and she was mother of his children and next Peshwa Nana Saheb (Balaji Baji Rao). The infamous Raghunath Rao was also their son.
Nothing much is known about Kashibai but with the little sources available, it's mentioned that she was an educated lady as she sent letters to her sons. She is known to be a really supportive, loyal and devoted wife to Peshwa Bajirao. She cared for her step son Shamsher Bahadur when his biological mother Mastani Bai died and she cared for him like her own son.
Mastani
The origins of Mastani are not clear, some say she was daughter of Raja Chattrasal (born from a Persian concubine). In Tarikh-i-Muhammadshahi, she is mentioned as a Kanchani (a skilled artist who used to perform and that would make it difficult to think she was a royal blood). She was well versed in sword fighting, horse riding, dancing and music. She used to accompany Peshwa on his battles but it's not clear whether she participated directly in wars or not. They had a son named Krishna Rao who was later renamed as Shamsher Bahadur.
Kashibai was the first and official wife of Peshwa Bajirao. She was Peshwen Bai and her status was exalted though one letter of Baba Phadnis mentions that there was no difference between auspicious Kashibai and Mastani, maybe hinting that the women didn't argue with each other or maintained cordiality.
So origins of Mastani is unclear but she was a strong companion of Peshwa Bajirao. Allegedly Mastani was blamed for some habits of Bajirao like drinking wine and eating non veg which was unsettling in Peshwa House but no authentic information is available whether Mastani indeed was the reason of these habits or his warrior lifestyle in General.