When did Sandi start chewing the feathers? Was it just after the baby was born or just recently after the feathers were trimmed?
There could be lots of different reasons here, its just finding the right one.
If the biting started just after the baby was born, Sandi could be feeling jelous and nervous of the new baby and change within the household, but you say the routine has not changed for Sandi so Im not sure on that one.
Was the trim done by an experienced Avian vet? If the clip was not done properly, it maybe could have left Sandi with some irratation around the flights but that would not explain the chewing of the tail feathers.
Do you bath Sandi regular? You could bathe more often then mist down with some Aloe vera juice to soothe the skin.
Make sure to take extra care if you bird has no flights, Ive had a bird lose flights and I had to make sure he had no risk of falling from ay kind of height as this would be very dangerous so keep Sandi at a low level if possible.
If you have not done so already, I would go back to the vets and have some tests done just to rule out anything medical as well.
Other members may be able to help you more, I do hope things improve and you can work through this, please dont think of re homing Sandi, it could make things worse not better.
Caroline.