Some TMNT stuff really isn't for little kids.
Shen first met Yoshi when he was meditating on his anger near a river, and he accidentally fell in when startled by her presence. The two felt an immediate attraction, and eventually married. She gave birth to four sons over the next few years, and was happy with her life.
However, Oroku Saki resented Shen and her influence on Yoshi. When Yoshi enraged him with open criticism of his hunger for power and bloodthirstiness, Saki sent a pair of Foot ninja to Yoshi's home to murder his wife and children. Shen was struck down by them, but Yoshi returned home just in time to save their four sons from a similar fate. Her dying words encouraged Yoshi to take their four sons and run, and she made her husband promise to not confront Saki in a fight, but to remain alive for their sons. Yoshi kept that oath, and he managed to hide for eleven years until they all were eventually tracked down and killed by Saki.
After their reincarnation and mutations, Splinter revealed to the Turtles the story of his former life, including Tang Shen's death. Leonardo sometimes dreamed of Tang Shen, which for him was a confirmation of this story. When Leonardo was brainwashed into turning against his father and brothers, Tang Shen's spirit appeared in his dreams and tried to guide him through his brainwashing back to his family. He became dependent on her guidance, subconsciously not trusting anyone as much as her.
When he was recovering in Northampton, she visited Leonardo again to help him through the alienation he felt towards his family. She also visited her husband Splinter for the first time, and reassured him that he had not failed Leonardo.
She also appeared after Donatello was savagely beaten almost to death by Rocksteady and Bebop. She gave him some advice, and assured her son that whether he chose to live or die, he wouldn't be alone.
Shen later appeared during Teenage Mutant Ninja Turtles/Ghostbusters 2, having called her sons to her. She imparts wisdom and love to the Turtles and thanks the Ghostbusters for helping them, and as they depart, she tells them to give her love to their father.