Sheen, who is also Richards' ex-husband, gave the temporary custody of his four-year-old twins Bob and Max, to Richards when Mueller was in a rehabilitation centre.
But Mueller has alleged that Richards' mistreated her children when they were in the latter's care, reports radaronline.com.
"It has been alleged that Denise not only physically abused the children, but neglected them as well. This is being taken very seriously by Denise. To say she is upset and disgusted by Brooke's actions are an understatement," said a source.
The source added that no proof to substantiate Mueller’'s claims has been presented. "But under the law, the claims must be investigated”," the insider added.
"Social workers might want to interview Denise's daughters and Denise will fight that. Her daughters don't need to be dragged into this drama. They are children who are very well cared for and loved."