Mo. Rev. Stat. § 527.040: Declaration of rights or legal relationships, right to obtain, when.

Any person interested as or through a personal representative, trustee, guardian or other fiduciary, creditor, devisee, legatee, heir, next of kin, or cestui que trust, in the administration of a trust, or of the estate of a decedent, an infant, mentally incapacitated person, or insolvent may have a declaration of rights or legal relations in respect thereto:

(1) To ascertain any class of creditors, devisees, legatees, heirs, next of kin or others; or

(2) To direct the personal representatives or trustees to do or abstain from doing any particular act in their fiduciary capacity; or

(3) To determine any question arising in the administration of the estate or trust, including questions of construction of wills and other writings.