Yes, you can call your sister "sweetheart" if it feels appropriate for your relationship. Many people use terms of endearment with family members as a way to express affection and closeness. However, it's important to consider her feelings and preferences, as not everyone may be comfortable with such terms. Overall, if it’s a term that resonates positively between you, it can be a sweet gesture.
His Sister's Sweetheart - 1911 was released on: USA: 6 October 1911
Let Me Call You Sweetheart was created in 1910.
Vugglemantis
Sweetheart
The cast of Let Me Call You Sweetheart - 1932 includes: Ethel Merman as herself
Those are nicknames that yo call your Boyfriend or Girlfriend. Like I do, Sweetheart.
Calvin and the Colonel - 1961 Sister Sue's Sweetheart 1-15 was released on: USA: 24 March 1962
Her name was Sweetie, and their Dad's girlfriend called her Sweetheart and she said that Sweetheart wasn't her name.
Let Me Call You Sweetheart - 1997 TV is rated/received certificates of: Australia:PG UK:12
Baby. Sweetheart. Hun. . Boo.babe
Call me my sweetheart
you call her your daughter's husband's sister