I have an ancestor who is a step child.

John Edward Bracken

I can not find the real father, do I seal the ancestor to the step father???

Brett .

@John Edward Bracken

John

I am just another 'lowly' User/Patron ...

[ And, I happen to be a Member of the Church ... ]

Short Answer:

'Yes', that can be appropriate, provided the Child was part of the Family with their Mother and Step Father.

I hope that this helps.

Brett

Brett .

@John Edward Bracken

John

Furthermore ...

Here are some "Knowledge Articles" in 'FamilySearch':

Policies for sealing deceased children to deceased parents

https://www.familysearch.org/help/helpcenter/article/policies-for-sealing-deceased-children-to-deceased-parents

How do I seal a child to grandparents?

https://www.familysearch.org/help/helpcenter/article/how-do-i-seal-a-child-to-grandparents

Can a child be sealed to parents when one of the parent’s names is unknown?

https://www.familysearch.org/help/helpcenter/article/sealing-to-parent-ordinance-when-name-of-parent-is-unknown

I hope these may help, somewhat.

Brett