Well, that is her choice. As long as she is not putting her children in any sort of danger, she has every right to date whoever she pleases. You may not like it, but one day... there may be someone your mother doesn't want you to date. Do you think you would like her telling you who you can and can't date?
Well, you just tell her how you feel about it in the nicest way possible.
However, don't expect her not to date him. It's her life, her choice... and if she doesn't see any good reason not to, then she has every right to date him.
i get what ur saying but my real dad past away and shes still married to my step dad. and now she wants to hook up with my real dads old best friend