Well, IF IF IF Zara IS a dog-aggressive dog,
many DA dogs have a lil trouble with shy dogs. (that is, IF IF IF Bella IS a shy dog).
siblings can and do fight as adults, too. So can mom/daughters, etc. Two of our pals have dogs from same family and those two dogs can NOT get along at all.
Even if your mom had never added Zara, Bella would still be a shy dog (or "unsure" dog as you call it). IF Bella is a shy dog, i mean.
it's genetic. But a shy dog growing up alongside a DA dog, is more likely to cop some of that dog-aggressive attitude. ---but that can be fixed, if Bella has dog-aggression going on, there is huge reason to hope for vast improvement in that.
Dog-aggression is also genetic, IF Zara is a DA dog. Zara can't help it either. It will be great, once someone finds a way to walk Zara, THAT can only help Zara be less crazy. Even a 'normal' border collie can go nutters if they dont' get enough physical and mental exercise. And Zara might turn out to be an extremely frustrated 'normal' dog, who knows?
Also, in the litter box, is no way to spot a dog-aggressive dog, no way for your mom to know. But, it is a LOT of border collie in one house, THREE border collies does require a person who LOVES training dogs!
But dog-aggression doesn't manifest til they are about 9 mos old, or even later if dog is not around unknown dogs much...no way to spot it in the puppy. I once read an article on how to spot a DA dog in the litter box, but, i am not entirely sold it can be even be done.
Shy dogs DO show up in the litter box, and soon as they are old enough to duck away from an unknown human, they will. It can be missed, though, if you aren't looking for it. Lotsa ppl don't really "read" what baby dogs are doing or "saying", they are admiring their markings, etc etc.
And usually, once a shy dog "knows" someone, they are usually okay with ppl they "know".
Some shy dogs are only mildly shy,
some shy dogs are severely shy dogs.
I CAN'T WAIT TIL YOU GET THEM EVALUATED!!
Either way, both Zara AND Bella can be helped to become their best possible selves! I really think, Tif, that you will be most pleased with improvements!! It's actually kind of exciting watching all the changes as they come along. It might be a lil bit, "Three steps forward, one step back" now and then,
so don't get discouraged, don't give up, both Bella AND Zara can become better and better and better!!
You shoulda seen MY dog when i first got him, an unholy nightmare, cujo dog!! Totally insane, did this whole wild-eyed, screaming, foamy mouthed, leaping and breeching monstrous reaction, was reeeeeeeeally ugly to see it.
and Buddy bit and growled at eveyrone he met, too. Was total gangsta, of epic proportions....reacted at unknown dogs from 400 feet away!!
he loves all humans
and he only reacts to unknown dogs at about 5 or 10 feet away, if he reacts at all
. Now when buddy reacts, it's just a bark bark! that's it. No more demonic displays of full on insanity and thrashing about now.
Buddy even has several doggie pals now!!
Buddy is not "cured" of his dog-aggression, but, he is well managed now. Took a while, but it has been a fascinating journey!! I have learned a lot.
I have made mistakes along the way,
but my dog always stands ready for me to try again.