There is certain thing called AGE . When betrayal happened mishti was not born but today she is adult who knows what is cheating.
Mauli has forgiven kunal for betrayal and mishti has may be forgiven kunal over the years BUT mishti is clearly unable to forget huge betrayal which her mother faced once upon a time in life .
Fact is mishti is not forgetting betrayal of her sperm donor because being grown up sane woman she understands what is betrayal means. Mishti understands how much her bio father and Nandini aunty were wrong.
For mishti , her mother was right and her bio father and nandini aunty will always remain wrong.