Proceedings have also been initiated against Conservative MP and chair of Westminster’s Northern Ireland Affairs Committee Simon Hoare, who retweeted Mr Beattie’s joke.
Patrick Higgins, from Donnelly and Wall solicitors in Belfast, told the PA news agency that letters had been sent to both men seeking remedies for their actions.
“Today we instigated proceedings on behalf of Mrs and Mr Poots against Mr Beattie MLA and Mr Hoare MP, who published this tweet,” he said.
“This post constitutes a grave and unwarranted attack on my clients’ reputation.
“As a couple of 36 years, married for 33, it has caused my clients profound distress.
“As a lady who has recently retired after dedicating her life as a nurse to caring for terminally ill children, it is wholly inappropriate for any persons to make such comments and the publication has caused immense hurt to her, her husband and their family.”