actually its not very clear in the course of the show why these two families have rivalry.. as much as they have told.. manpreet grandfather was killed by bajwa's.. they cheated him to an invitation to their place and killed him... in the start of the show there was charan ( manu's dad) and harjeet's father whose character mukesh khanna played.. they wanted to end this rivalry.. but jagan and harjeet being hot headed they always got loggerheads.. so in one such fight, charan got shot by mistake and died.. since charan was the shah.. the only successor to him would be either jagan or is charan has a son then he becomes the shah... charan's mother wanted amba to give birth to a boy .. but it was a girl.. amba lied that manu was a boy so that jagan doesnt become a shah.. cos if jagan becomes shah then it will increase the rivalry and many ppl of the pind will lose their lives.. harjeet's father also died later in the show.
so manu was raised as a boy... but when he turned 8.. he came to know that he is a girl.. initially preet is angry with her mom but later she understood why amba took such a step and promised to be a boy always and fulfil her fathers dream of ending this rivarly..
coming to mohini.. she almost came into the show towards the start of the leap.. there was one guy who came into amba's life.. infact amba fell in love with him.. but he got killed.. mohini was friend and she loved him too.. so she wanted to take revenge from amba.. she ended up getting close to harjeet to revenge amba.. and she was the one who shot harjeet's son raman... one day she got two kids with her and told harjeet she has noone to help her raise her kids.. she needs money etc.. harjeet got carried away.. dont know wether they r her own kids or not.. it was not clear..