Amount of financial support only makes a difference if the student has spent the same amount of time with each parent over the previous 12 months; in that case, it is used as a tie breaker. If your older daughter has spent more time with you than with your ex-wife over the previous 12 months, and your step-daughter has spent more time in your household over the previous 12 months than with her biological father, than the FAFSAs for those two students would report any 529 accounts that you own as parent assets, regardless of who the named beneficiary is. Distributions from any 529 account that you own to either of these two students will not count as income to the student.
If your younger daughter has spent more time with your ex-wife over the previous 12 months, than any 529 accounts of which you are the owner will not be reported on your younger daughter's FAFSA as an asset. However, any distributions that you take from a 529 account that you own, for the benefit of your younger daughter, must be reported on her FAFSA as untaxed income to her. Note that with the new prior-prior year income and tax reporting for financial aid, distributions of this kind generally made in spring of the sophomore year or later will not have an impact on financial aid (assuming the student finishes college in four years).