How do you handle step-siblings &/or step-children? How are they entered and where?
For the step-child relationship, once you’ve attached a parent you can modify what kind of relationship it is.
You go into the edit person screen for the child, select the ‘Parents’ row and then update the relationship type. (See below screen capture.)
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@KimberlyGreen did a great job of telling you how to do it BUT you need to know that RM does NOT SUPPORT step-parent/ step-child relationships nor adoption etc. 
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In my hubby’s case, his Mom was married 4 times-- if I set up my database as above, my hubby then has 4 FATHERS listed in the Kinship List ( instead of 1 Father and 3 spouse of wife)-- that also means that hubby also has a whole bunch of GRANPARENTS and AUNTS AND UNCLES ( from the step-fathers) - the Relationship Calculator shows FATHER/SON relationship for hubby and his 3 stepfathers (as well as extra Grandparents etc)-- the narrative reports will also list hubby as a child of the stepfathers-- That’s the bad news…
Now the good news is that it will show up on the child’s page ( not the step-parents) page on Ancestry–adoption does also
Might want to consider using just a Customized Fact and share it or a Customized Association when they get it set up right as Associations will not transfer in a gedcom or drop and drag right now–so it won’t show up on Ancestry BUT a Customized Fact will show up if uploaded thru a gedcom-not sure abt Tree Share…
edit-sorry forgot to say that if you share a fact-- on Ancestry the fact does not show up on the page of the person you shared it with–just the principal person…