Is it conflict of interest to have lawyer who represented family to be hired by ex?

It is a conflict of interest unless the lawyer has the informed written consent of the former and present client. Complaint to your state bar association.

I am not licensed to practice law in Ohio so can offer only general information and resources. Ethical issues tend to be complicated and technical. I would not recommend a bar complain until you have confirmed that an ethical violation has occurred - or at least until you have a good reason to believe so. It isn't clear from your description that a violation occurred. Your fiance is not a former client of the lawyer.

You may wish to review;

http://www.law.cornell.edu/ethics/oh/narr/

Pursuant to Rule 1.7(a), a conflict is present with respect to acceptance or continuation of representation if
(1) the representation of that client will be directly adverse to another current client; [or]
(2) there is a substantial risk that the lawyer's ability to consider, recommend or carry out an appropriate course of action for that client will be materially limited by the lawyer's responsibilities to another client, a former client, or a third person or by the lawyer's own personal interests.
These two types of conflict (direct adversity and material limitation) can be overcome only if all of the factors set forth in division (b) are present:
(1) the lawyer will be able to provide competent and diligent representation to each affected client;
(2) each affected client gives informed consent, confirmed in writing; [and]
(3) the representation is not precluded by division (c) of this rule.
1.7:210 Basic Prohibition of Conflict of Interest

Excerpted and adapted from Arthur F. Greenbaum, Lawyer's Guide to the Ohio Code of Professional Responsibility § 5.3 (1996).

I recommend you tell your fiance to contact an attorney to go over the facts of your case, or the state bar association. I also recommend you encourage your fiance to do this for himself. This is his case. You may want to help him, but it is important that he take primary responsibility for finding out his legal rights.