Okay, I have a bunch of questions about the damage using Nerva does to a Sister's realm. (English Void version, in case that matters.)

1) Approximately how much damage can a Sister's realm sustain before the predators appear? How much until the Scorpio?

2) Does feeding the Sisters with the Donor glyph damage their realm?

3) Does poisoning a sister with bad Nerva damage her realm or is there some separate health total? Does the Vampire Glyph damage her realm?

4) Does fighting a brother with glyphs in his domain damage the realm of the Sister he protects? Does killing a Brother damage his Sister in any other way?

5) Ire mentions glyphs damage a Sister's realm when they are used, Donor being the least damaging. Will planting a full garden (11-10 trees in some places) result in noticeable damage?

6) Approximately how much protection does 100% Silver in your hearts provide a Sister's realm?

Predators always appear. A realm can stand about 4000-5000 of colour (you may increase the limit by placing Silver in your heart, so that the colour does less damage). A serious damage is about 50 %, and that's approximately the point at which nasty monsters begin to appear. So you have to use about 2000 drops of lympha to achieve this. Yes, it does. It damages the real even more than usual, because the sister's favourite colours damage her realm more than others. Note also that feeding a sister does twice as much damage as reviving a tree with the same amout of colour. The trees have built-in multiplier of 0.75, while feeding Sisters does x1.5 damage. As far as I know, every single act of using the colour in sister's realm in any possible way - damages her realm. No, it doesn't. All these fights are considered as if they were happening "outside" of the realms. Let's do some calculations. As reviving a tree does only 75% of basic damage, actual "limit" is around 6000-7000 drops (that's the amount you need to use to kill the realm completely). "Serious" damage is a thousand or two. Trees in Nameless Sister's garden have capacity of 40 drops of lympha, and there are 11 of them. Which results in 440 damage (actually, it is going to be 20% more if you use her favourite colours, Green or Gold). If you have 100% Inspiration (Violet), it is going to be 33% less, as your Donor will become 50% more effective on the trees (you'll use 27 drops instead of 40). If you have 100% Cold (Silver) then the damage is going to be two times smaller, because each drop of these 27 does two times less damage. It is purely theoretical, but if you have both characteristics at maximum, the damage will be equivalent to 147 drops of color, which is a very small damage (given that you cannot use this garden for 10 cycles after that).

Now, let's see what happens in Ima's Garden, with 75-drop trees. There are also 11 of them. So, we get... 825 damage. Pretty serious... if you have 825 drops to waste at once, that is. Again, full Cold and Inspiration reduce the damage thrice, to an equivalent of 275 drops.

Yani's garden also has 11 trees (it is coincidence, honestly). 5 trees hold up to 75 drops, the rest 6 can be revived with as much as 125 drops, which gives 1125 possible damage (375 if you have 100% of Inspiration and Cold). Well, theoretically possible, if you just happen to have 1125 drops you have nothing to do with. As the limit is more than 6000 drops, you cannot even damage the realm to 50% this way unless you start doing it on 5th cycle and never stop. Remember, only after 10 more cycles you will be able to plant the garden again, no matter how much you want to kill it!

So the damage may be quite serious in the farthest gardens, but nothing to worry about in those closer to your starting point. Trying to set the girl free is probably the most serious damage you can do, as most of them require 2000 colour (Ole and Yani require 3000 each), used with the multiplier two times bigger than the multiplier trees use. If you try feeding the girl past the first two hearts without any Silver of Gold in your hearts, the realm will probably become unusable long before you are done.
Why approximately? Each drop's damage is halved. Exactly 50% of what it was without a single drop of Silver in your hearts.

Thank you so much for such a quick, thorough response! You devs are great!

So just to make sure, you said using a Sister's color in her realm does 20% more damage, right? Feeding her those favorite colors does x1.5 (for feeding a Sister) and x1.2 (for her favorite colors). So that's 70% more damage! No wonder my gardens get infested so quickly!

Wow, it didn't even occur to me that giving Sisters color could hurt their Realm until I just asked. Good to know! For future reference, how much of a color bonus does 100% Gold give? I think I figured it out to be something like a 70% boost, but all of a sudden I don't trust my math.

Both Violet and Gold increase efficency by 50%, i.e. the same amount of actual color produces 50% more effect. Violet increases the effect for trees, and Gold for Sisters.