Both the Gramps Relationship Calculator (Tools->Utilities->Relationship Calculator...) and Narrative Web support displaying relationships for people such as 'stepmother', 'stepfather', 'stepbrother', 'stepsister' and 'step-sibling'.
There have been several discussions in the gramps-devel mailing list and in bug reports about the correct interpretation of these terms. This page provide some evidence for interpretation.
It is understood that some people feel personally uncomfortable with referring to someone who has (or had) married their father(mother) as their 'stepmother' ('stepfather'), particularly if they are not living in a household with that person. However, the authoritative definitions of the terms do not restrict the use of the term to people in the same household. Gramps uses the terms in accordance with the definitions. If you do not want your Narrative Web report to show people as step-relations, then you can ensure that you have unchecked the 'Include half and/or step-siblings on the individual pages' option in the Advanced Options tab. If you want some specific person to be shown as a step-parent, then you can include the person as a child in the family in which the step-parent appears. Gramps also follows what is apparently the American usage of disregarding the order of marraiges when deciding whether someone is a step-parent.
Definition of 'stepmother'
What is the definition of 'stepmother'? Note that the definition of step-father is mutatis mutandis similar, we just choose one to focus on.
The Oxford English Dictionary  says "A woman who has married one's father after one's mother's death or divorce".
Merriam-webster  says "the wife of one's father when distinct from one's natural or legal mother".
The American Heritage® Dictionary of the English Language in  says "The wife of one's father and not one's natural mother".
Collins English Dictionary in  says "a woman who has married one's father after the death or divorce of one's mother".
Analysis of definition of stepmother
The definition can be divided into two parts, the wife of one's father, and a time element.
one's father’s wife and not one's natural mother
A stepmother is not genetically related to oneself. There is no common ancestor, the relationship is only by marriage.
The Gramps Relationship Calculator (Tools->Utilities->Relationship Calculator...) works by a recursive algorithm starting from a person and going up through all parents  to find a common ancestor. Since a person and their stepmother do not share a common ancestor, they will appears as 'unrelated' in the calculator. It is not proposed to change this outcome, because looking at all marriages for each person (particularly recursively for all ancestors) would be very expensive in run-time, for little benefit.
There is an exception in the Relationship Calculator. If a person is recorded as a child in the family which includes the step-parent, for example because the child was adopted by the step-family, then the relationship calculator will show the step-parent as stepmother or stepfather of the child.
after one's mother's death or divorce
Some, but not all definitions of 'stepmother' include the requirement that the step relationship occurred after the relationship between one's natural parents.
It appears that the American usage (Merriam-Webster and American Heritage) ignores the order of the marriages, and Gramps also does so, partly because this is simpler, and partly because the relevant dates may not be present in the Gramps database.
Definition of 'stepfamily'
'stepfamily' is not, strictly relevant to this discussion, because both the Relationship Calculator and Narrative Web are only displaying the relationship of one person to another, not whether they are in any particular type of family or not.
The OED definition of 'stepfamily' is: "A family with one or more stepchildren; a family in which at least one of the adult partners has children from a previous relationship or marriage (though not necessarily living in the same household); (also) members of such a family."
Wikipedia  has quite a long discussion around the meaning of stepfamily, including "Children from a stepfamily may live with one biological parent and visit their other biological parent, or they may live with each biological parent for a period of time" and "The traditional and strictest definition of a "stepfamily" is a married couple where one or both members of the couple have pre-existing children who live with them" and "Some people also apply the term to non-custodial relationships, where "stepparent" can refer to the partner of a parent with whom the child does not live. The term is not generally used (but can be in individual cases) to refer to the relationship with an adult child who never lived in the home with the parent's new partner." However, none of these definitions refer to formal dictionary definitions, and indeed it is hard to see how the references actually substantiate the descriptions that are supposedly derived from them. For example "adopting.org" points out that because some children do not legally "reside" with their fathers, government and academic research do not include them and they are ignored and uncounted. however, the context makes it clear that they are talking about stepfamilies, and otherwise why would it be relevant that such people are not counted.
There have been extensive previous discussions of step-siblings.
- 2012 discussion (the proposed work cannot be completed) 
- 2010 discussion 
- 2009 discussion 
- 3189: Error in calculation of step siblings.[CLOSED]
- 6068: Narrative Web reports two people as step family when one of the parents is unknown for both.[RESOLVED]
- 5859: Stepsiblings are NOT correct.[RESOLVED]
- 6115: RelationshipCalculator get_sibling_relationship_string gives unexpected results when sibling type is unknown (This is a problem in Relationship Calculator, not Narrative Web).[confirmed]