Why did Ngola Mdandi king of Ndongo send his sister Njina to negotiate with the Portuguese

Because she was the successor to the throne. She was an outstanding negotiator and a powerful military general.

Ngola Mbandi, King of Ndongo, sent his sister Njina Mbandi to negotiate a peace settlement with the Portuguese because she had shown exceptional negotiation and diplomatic tactics and had won significant concessions from the Portuguese by the end of her trip to Luanda.