Greetings, Scriptor.  Recalling my #434605:

Antonette is to Teuntje as
Ysabella is to X
Margriet is to Y
Kateryne is to Z

What are X, Y, and Z?

Thank you,
Archae0pteryx

Hello, dear Archae0pteryx! Glad to hear from you again!

These are my suggestions for the possible nicknames:

Ysabella: Ibel, Beletje, with the latter being more intimate through
the diminutive suffix -je.

Margriet: Griet or Grietje, with the latter being more intimate
through the diminutive suffix -je.

Kateryne: Katrijn, Trijntje, Trijn, Tryntje. The latter may be of
particular interest for you, since it contains an "y", just like the
individual spelling of the original name. However, the "y" of the
medieval "Kateryne" might also be a contraction of i and j rather than
an actual y, so that Trijntje could be more fitting in that case.

Concerning the age of those nicknames: Research in genealogy lists
confirmed that all these names have long traditions and date back
several centuries. For "Katrijn", for instance, I found one example
from before 1425.

I hope that this will be helpful!
All the best,
Scriptor

Perfect, as always, friend Scriptor.  And in record time, too, I
think--no overnight wait for time zones to catch up.  I must have
caught you on your day off.

The writing is going well, taking some interesting turns, developing
new dimensions.  Hope the same is so for you.

Best regards,
Tryx