The Michigan statute of limitations for professional liability (excepting doctors) is 2 years from the date of malpractice or 2 years from the date of last representation in the matter in which malpractice is being claimed, whichever is later.
The limitations statute applies to your time limit in bringing a claim, it does not apply to the age of any evidence, such as e-mails. So evidence, even if older than 2 years, can be used as evidence in a case as long as it is relevant and has a proper foundation (and complies with the Rules of Evidence).
It seems your question may really be whether more recent e-mails can be used to extend the statute of limitations? If that is the question, then the question is whether same represent legal advice in the matter in which you are claiming malpractice. You would be best served by having a lawyer who handles legal malpractice cases to review any e-mails, or other work/advice, from the attorney to give you an analysis. If you are saying the advice was back in 2010 and the lawyer had done nothing on the matter since then, and it is simply you sending e-mails to the lawyer more recently, it is unlikely that the statute of limitations will be extended by unilateral action by you; however I again recommend you consult with an attorney who can review all the facts and possible evidence to give you more complete advice.
Perhaps you will also want to speak to a lawyer that handles estate matters and see if there is any claim against your sister and/or her children for contribution as to amounts you paid in taxes that were not your responsibility.
