and E.M. ?" Those last two letters might stand for the initials of my husband's name--his true name--Eustace Macallan.

In this case the first letter ("S.") in all probability indicated _her_ name.

What right had she to associate herself with him in that manner?
I considered a little--my memory exerted itself--I suddenly called to mind that Eustace had sisters.

He had spoken of them more than once in the time before our marriage.

Had I been mad enough to torture myself with jealousy of my husband's sister?
It might well be so; "S." might stand for his sister's Christian name.