Sunaina was attacked by Karthik's goons and left for dead. But unknown to them, she was rescued by some people. When she came to, she lost her memory and had to be shifted to a welfare home where people called her Aditi. Two years passed and Sunaina's memory returned one day. She recalled her attack, but did not know who was behind it. When she tried to contact her husband, she found out that some woman named Simar was acting as Sunaina upon her hubby's instructions. She did a background check, and came to know Simar did all this for her younger sister who is named Roli.
Having come upon this information, she cleverly staged a scene where Roli would bump into her. Once she got acquainted, she let Roli know Sunaina's (her) story and indirectly encouraged her to investigate and find out the culprit behind the attempt on her life. She couldn't go to the police since she doesn't have enough proof (she was attacked before she could complete her investigation), and she couldn't go and investigate herself since people at Navjivan hospital would recognise her.
Owing to the above factors, she needs Roli to find out the man behind it all, so she could go back to her rightful place in her family. And she chose Roli since Roli too wants her didi back, so she'd have an intetest in solving Sunaina's mystery.
Half-baked, half-burnt and crusty as it might be, that's my theory. Cya.