Lindo is Marta's son, born from her forbidden relationship with a vampire. In episode 9, Lindo reveals that the shame of baring a Dhampir child was too much for her to bare. She refused to see him as her son, saying that her real son was dead, eaten by a monster.
Nesta was Marta's vampire lover, and the father of her Dhampir son, Lindo. Marta appeared to love Nesta once, but came to regret their relationship after Lindo was born.
Nesta used his relationship with Marta to find out about the grimoire, which was inside Ritsuka. It is later revealed that she told him willingly.
Maria was Marta's sister. They were presumably close, and shared a loving relationship. They were close enough that Maria took it upon herself to raise her son after she died. It is later revealed by Nesta that while Marta did love her sister, she also resented her for being their father's favorite daughter. This eventually drove her to betray the Tachibana family.
However, it is presumed that she regretted her decision after Lindo was born. Also, if Maria bore any ill will towards her sister, she chose not to take it out on her son.
Ritsuka was Marta's niece. It is possible that Nesta used his relationship with Marta to find out about the grimoire, which was inside Ritsuka.