I agree with Mephiles97 that no one knows for sure. But I mostly think that she could be dead. Because even if she had a divorce, Carly and Spencer would've mentioned SOMETHING about her. または she might have even visited them.
Nobody knows what happened to her mum and if they claim to do so then they probably lie. Nothing is know about her either, since Dan leaves out details of her parents being absent. Jerry Trainor was asked what happened to her and he responded, 'I don't know, shopping I guess', while laughing.