This is a question often asked on this site. Your fiance's children are too young to make their own decisions regarding custody. Between 11- 13, they can voice their choice. However, it doesn't hold much sway until the children reach 14.
More specifically, the exact language of the custody code is below.
(5) In all custody cases in which the child has reached the age of 14 years, the child shall have the right to select the parent with whom he or she desires to live. The child's selection for purposes of custody shall be presumptive unless the parent so selected is determined not to be in the best interests of the child. The parental selection by a child who has reached the age of 14 may, in and of itself, constitute a material change of condition or circumstance in any action seeking a modification or change in the custody of that child; provided, however, that such selection may only be made once within a period of two years from the date of the previous selection and the best interests of the child standard shall apply.
(6) In all custody cases in which the child has reached the age of 11 but not 14 years, the judge shall consider the desires and educational needs of the child in determining which parent shall have custody. The judge shall have complete discretion in making this determination, and the child's desires shall not be controlling. The judge shall further have broad discretion as to how the child's desires are to be considered, including through the report of a guardian ad litem. The best interests of the child standard shall be controlling. The parental selection of a child who has reached the age of 11 but not 14 years shall not, in and of itself, constitute a material change of condition or circumstance in any action seeking a modification or change in the custody of that child. The judge may issue an order granting temporary custody to the selected parent for a trial period not to exceed six months regarding the custody of a child who has reached the age of 11 but not 14 years where the judge hearing the case determines such a temporary order is appropriate.