Could you provide more information and/or a picture? Are these "veins" of water on the glass? or around the Sash? What type of windows are they? and do these "rivulets" only appear when it rains?
I would like to help but without being able to see the leak, etc. it will be difficult. Do you have or can you take pictures of a leak "in the walls all around the window", both inside and out?
In order to help I need facts, not conclusions such as "It must be an internal leak. But we have sealed around
the windows with silicone".
Lastly, For your information I am a professional Carpenter/Business Owner with over 40 years working on old houses.
I finally got around to taking the attached photos. For all of the windows that do this, we have (1) scraped and sanded and repainted (2) applied KILLS treatment. For some of the windows, we have also done extensive caulking/sealing around the outside window casement.
It seems to vary - the condition returns after a short while, on some windows; on others not. When it does return, it starts small, then grows.
What do you recommend?
The pictures help but there is still much unknown about the construction of the house.
Looks like a stone or brick wall with plaster applied directly over the stone/brick? Water is getting into the wall, likely from the outside. It would only take a small crack or hole in the pointing, stucco, etc.
If you haven't already, I would check the exterior head and sills of the window. Again, look for any crack, hole or opening.
If there is no obvious hole, crack etc in the exterior. Then there are two other possibilities- the roof, including the top and sides of the parapet. Again, you would be looking for a crack or small hole that wind driven rain could enter. If you had a large hole then the wall would be getting wet all the time.
Last possibility would be "rising damp" from the ground, up through a rubble stone foundation. This should show in the basement walls if they are "parged".
Hope this helps.