Bindusaar will marry Dharma because

From yesterday epi we can clearly see that how Dhrama is excited to see Karthikey and she feels he is better than Bindusaar and Alice gets attracted to Karthikey so Bindusaar for his sister's happiness he will marry Dharma  so that she can never come in between them 

Am giving from my POV 

From what I have seen from Ekta shows and all 

That the brother will force to be married to the person who comes in between his sister and her lover

At the end of the day, the fact these historical kings have had more than one partner makes it hard to believe that they could've actually loved one of them as they are trying to be depicted in the stories and serials. 

if it weren't for the pretty outfits and nandini/bindusar- nandini/dharma maternal bond I don't think I would be able to watch this.

Who else wants for Chandra nandini to change up history a bit by opting out of Bindusar having more than one queen?  It's not like the serial is an exact depiction of history, there have been changes made... 😉