Not so wild after all, Grishka,
and I can see how it could have happened!
Though I still have much to learn about the composer,
I recall what I looked up and learned about Carl Loewe's life.

He married twice; his first wife was indeed
a lady with the maiden name/ family name "von Jakob," and
the redoubtable "Talvj" would be her sister.
Before the wife died, far too young,
she brought their son into the world.
I forget at which moment the sister-in-law
became a surrogate mother to Loewe's motherless son.

But I do recall that when "Talvj" and her husband set off
for her husband's native United States,
Loewe's son was part of the household
and he was raised very much as their child.

Loewe was very busy with his career, and it was a while
before his second marriage.
Between one thing and another,
father and son parted company early in the son's childhood.
And they remained distant even though they were not estranged,
just by being in different households in different continents.

Yes, it could very well have been the sister-in-law,
a woman of very respectable birth in a time and place
when women writers were still discriminated against,
and a writer who found covert ways to get around
the obstacles in the way of her writing.