Mastani Bai
Mastani Bai’s origins are highly debatable but the popular speculation is that she was daughter of Raja Chattrasal born from his Persian concubine. In Tarikh-i-Muhammadshahi, it's mentioned that she was a Kanchani (skilled in many arts like dancing, music, riding etc.).
She was second wife of Peshwa Bajirao and they got married shortly after his help to Raja Chattrasal in his Bundelkhand campaign against Bangash.
Polygamy was a known norm in that era although her origins was the reason that made Peshwa’s family highly uncomfortable. It's said that they blamed Mastani for his habits of drinking and eating non-veg although there are no evidences available which suggests that Mastani encouraged him to do it or it was just his warrior lifestyle. So, technically Mastani can't be blamed for being a homewrecker although Peshwas rarely were polygamous with few exceptions because they technically were not kings but also polygamy was not forbidden to them.
Bajirao kept loving his wife Kashibai and they had many children together. When Bajirao and Mastani died, Kashibai brought up their son Shamsher Bahadur like her own son.
The movie actually did damage to Bajirao and Mastani by showing them hurting Kashibai by cheating behind her back which actually didn't happen in history. Kashibai and Mastani kept their distance with each other. Bajirao was a warrior who mostly remained in battlefield and hardly had time to think about kitchen politics. Mastani was briefly house arrested by Chimaji Appa because they suspected that Mastani was doing harm to Bajirao by making him indulge in unorthodox lifestyle which was not expected from a Peshwa. Shahu Maharaj wanted Peshwa to remain happy at any cost as he was the one who was winning wars and keeping the Empire intact and therefore was not in favour of keeping Mastani away from Bajirao.