What is the religious ruling on a paternal uncle's widow communicating daily and via text messages with her husband's brother?

A man corresponding with his brother's widow and speaking with her unnecessarily is blameworthy (munkar). His seclusion with her is even more blameworthy. The matter becomes uglier due to her being his brother's wife and the mother of his children, and "maintaining ties of kinship is part of faith."

Both of them must be advised and reminded of Allah with wisdom and good admonition. It is preferable that the advice come from someone whose words are likely to be accepted by them. He can be guided to marry her if he desires; otherwise, he must completely avoid her and sever all ties with her, to protect his religion and honor.