Especially, she almost got Thomas killed. If you really think about it, she might indeed need to face legal responsibility.
Henry could also be considered the head of the Thomson family. Normally, he wouldn't get involved in the matters of the second branch, nor would it be appropriate for him to do so. Moreover, he didn't quite like Avery Clark himself. However, no matter what, she was still a senior family member to Robert Thomson and Emma Smith, a member of the Thomson family, and his sister-in-law.
To put it bluntly, no matter how outrageous this sister-in-law's actions were, Robert calling the police with one phone call was a slap in the face that might have been too loud!
And why did Avery do this? Wasn't it because of the conflict with Emma and her mother?
Even if the mistake was hers, shouldn't Emma and Mrs. Smith bear some responsibility for these conflicts?