ddo you mean ther emay be other elements in the same sequence, but you
just want to group related-article?

In which case (if there are no sibling text nodes)  something like

<xsl:for-each-group select="*"
group-adjacent="exists(self::related-article)">
  <xsl:choose>
    <xsl:when test="current-grouping-key()">
    <related-article><xsl:value-of
select="current-group()"/></related-article>
</xsl:when>
<xsl:otherwise>
  <xsl:copy-of select="current-group()"/>
</xsl:otherwise>
</xsl:choose>
</xsl:for-each-group>