In an emotional letter unsealed today, Catherine Greig’s twin sister, Margeret McCusker, asked a federal judge to be lenient when sentencing Greig because she had grown up with an alcoholic father and had no role in mobster James “Whitey’’ Bulger’s crimes.

“Cathy never possessed an evil bone in her body and was never involved in Jim Bulger’s activities,’’ McCusker wrote in the letter sent to US District Judge Douglas P. Woodlock.

The documents were unsealed one day after Greig was sentenced to eight years in prison for helping Bulger elude law enforcement for 16 years and on the day that her attorney filed a brief legal document seeking to overturn her conviction.

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Kevin J. Reddington filed the one-paragraph “claim of appeal’’ in US District Court in Boston where Greig was sentenced Tuesday by Woodlock on charges of conspiracy to harbor a fugitive, conspiracy to commit identity fraud, and identity fraud.

“Now comes the defendant, Catherine Greig, in the above numbered indictment and does hereby claim an appeal of the above captioned conviction to the United States District Court of Appeals,’’ Reddington wrote.

Reddington didn’t immediately return a phone message seeking comment.

In the letter, McCusker explained she did not know why her sister went on the run with Bulger. McCusker said her sister had “a sense of duty to care for people that most of us do not have. She has touched many with her kind acts and her love for animals is unsurpassed.’’

McCusker stated that growing up in South Boston, her family had a “lonely life’’ because they were kept isolated from their neighbors because of a family secret—“my father drank to excess … We grew up in a family environment that did not interact with society well. ... My point is that growing up, Cathy did not have a strong father figure in her life.’’

McCusker urged Woodlock to note “the positive influence she had upon Bulger ... She is portrayed by the media and the US Attorney’s office as a person who was corrupt; that assertion is so far from the truth.’’

She added, “She did not participate in any criminal enterprise then or now. She is guilty of falling for someone that was involved in those kinds of things. … Cathy saw a side of Bulger that tough guys who worked for him never did.’’

McCusker’s letter to Woodlock was unsealed today along with letters sent by relatives of some of the people Bulger allegedly murdered during his years as an organized crime figure who was also working as an informant for the FBI.

On Tuesday, the federal probation department advised Woodlock that Greig should face a sentence ranging from 27 months to 33 months based on the nature of the charges, her personal history, and the fact that she had no prior record. Reddington urged Woodlock to sentence her to 27 months, while prosecutors argued the length of her prison sentence should be increased to 10 years.

Prosecutors argued that Greig should face a longer sentence because she had to have known about 30 guns Bulger hid in their Santa Monica, Calif., apartment. They also said she lied to court officials in California after her arrest last year by claiming she had no assets. She owns a home in Quincy and at the time had a bank account in Boston containing $135,000.

Leaning more towards prosecutors’ recommendation, Woodlock sentenced Greig to eight years.

Boston attorney Michael Kendall, a former federal prosecutor now handling white collar defense cases, said in a telephone interview there is likely just one issue Reddington wants reviewed – the eight-year sentence.

“Hypothetically, you can do that, but practically, I wouldn’t bet the rent money on it,’’ said Kendall of the firm McDermott Will & Emery. “It doesn’t sound like an appeal with a great chance of success.’’

Kendall said sentencing decisions by trial judges are rarely overturned, especially if the judge provides a legally justified rationale for increasing the length of a prison term beyond what the probation department has recommended.

In the Greig case, Kendall said, Woodlock’s decision seems based on solid legal grounds because she admitted helping Bulger – accused of multiple murders – to evade capture for a long period of time, 16 years.

“It’s certainly within the ambit of what you would expect,’’ he said. “I think he [Woodlock] stated very adequate grounds that any appellate court is going to see the logic of. Sixteen years is a long time.’’

Kendall said that under federal law, a defendant does have the right to withdraw a guilty plea, but he said such a request is rarely successful.

On Tuesday, Reddington said that Greig would get credit for the year she has been held without bail and earn good-behavior time, which would make her eligible for release in about 5 1/2 years. Greig is being held at the Donald W. Wyatt Detention Facility in Central Falls, R.I.

Reddington also told reporters Tuesday that Greig did not regret spending 16 years on the run with Bulger.

“She’s in love with the guy. If she could be with the guy right now, she’d be with him,’’ said Reddington. He said Greig did not believe that Bulger, who is scheduled to stand trial in November for 19 murders, was capable of such horrible crimes.

“She absolutely stands by her man,’’ he said.