One of Margaret Oliphant's great gifts was to draw characters vividly and truly, including those having devoted friendships with someone of their own sex - devoted either on both sides or on one side. It is not important to pigeon-hole these characters in modern terms as gay or straight or bisexual (and in fact many of these characters do end up marrying) - what mattered to Mrs Oliphant was to portray truly the emotional nature of the bond.
In Lady Isabella and The Sorceress the friendships are central to the story lines (though in the case of The Sorceress, it is a backstory being retold). Also in Lady William, though the characters are not exactly central, their relationship is important in furthering the story line.
In other novels the relationships are light-hearted minor story lines. There is for instance in Katie Stewart Lady Anne Erskine, who adores Katie Stewart as a child and keeps that allegiance her whole life. Or the two girls in Salem Chapel who become inseparable. And there is another minor character, Lord Millefleurs in The Ladies Lindores, who recalls enthusiastically his life in California among the rough miners.