Ann Duffy, of Plymouth, was also issued an indefinite restraining order against her brother's wife.
The 50-year-old had previously admitted harassment without violence. Prosecutors compared the case to an EastEnders' plot line.
Magistrate Graham Price said Duffy's conduct was "totally unacceptable".
'Saved from divorce'
Duffy impersonated her now sister-in-law Sandra in a phone call to Plymouth Register Office to tell them to cancel the ceremony.
She made the call on 5 November, 20 days before the wedding was due to go ahead.
The court heard she called her brother David Greatrex and told him: "You had better put this on speakerphone.
"I have saved you from a divorce. I have cancelled your wedding. Would you like me to send you the confirmation email?"
The couple contacted the police to say Duffy had impersonated Sandra and called off the ceremony.
'Cruel, vindictive act'
Prosecutor Will Rose said: "It is like a lift from an EastEnders' plot line, but it was the sad reality that confronted the complainants in this case."
It was a planned, deliberate, spiteful and vindictive offence”Magistrate Graham Price
He said it was "a cruel and vindictive act which left the couple distressed and distraught".
They were able to reinstate the wedding, which went ahead on the chosen day.
In a police interview, Duffy admitted she had a strained relationship with her sister-in-law who was looking after her mother.
She said she was trying to protect her brother from a marriage to someone she disliked and who she felt was taking away her mother.
Duffy said she only wanted to cause Sandra distress, not her brother.
Miss Rebecca Wood, mitigating, said: "She does not like her sister-in-law and there are ongoing issues between them.
"It is not an excuse. She intended to cause inconvenience and upset."
Magistrate Mr Price said: "It was a planned, deliberate, spiteful and vindictive offence."
Outside the court, Mr Greatrex said: "She was really jealous and kept accusing Sandra of trying to take her brother away."