You can ASK her to leave, but do not expect the police to toss her out when she refuses.

You will almost certainly have to evict her if she does not choose to leave on her own. And, if you lived in the house together while she was raising the kids, she was contributing to the household (and probably has continued to do so in some way)and that removes her as a "lodger."

As inferred by Mr. K., if she has physical custody of the kids, you'll also be booting out the kids.