If I am representing a client for estate planning, can I accept a referral fee from an insurance agent or an investment company I have worked with in the past for referring the estate business?

The answer traditionally would be no. Lawyers are not allowed to share fees with a nonlawyer or receive fees from a nonlawyer, see Rule of Professional Conduct 5.4(a). Another good reason not to enter into such an arrangement is it could create a conflict of interest under Rule 1.7 of the Rules of Professional Conduct. As part of the conflict of interest rules under the concurrent conflict of interest rules, a lawyer cannot represent someone if the representation would be materially limited by the lawyer’s responsibility to a third person or the personal interest of a lawyer. There can be a waiver under certain circumstances, but there has to be full disclosure.