maybe it was angry that the other one let the vet spay him/her

It's most likely that there are several different things going on here. I assume you had the older Bitch first. This means she was an only dog for some period of time. Even though she was willing to accept a puppy...and she may well be a friendly, social girl most of the time...the puppy has come to stay....AND she isn't a puppy so much any more (she actually IS still a puppy, but not so much to your Senior Bitch...from her perspective.) In a canine world, the youngster is starting to reach that stage where she MIGHT possibly start to make a bid for more of a leadership position, between the two of them. Your Senior Bitch may not be he Alpha here, even though she is older. Typically a true Alpha canine NEVER starts fights. They are so self assured in their position of leadership, that they frankly feel they have nothing really to prove to anyone. They rule with benevolence and good temper; usually satisfied to simply give "a look" or often just sort of "jolly" other dogs in to calming down an aggressive or stressed situation. There is some thought towards the fact that now the younger bitch DOES smell differently to the Senior, she no longer has the ability to reproduce. But, most Behaviorists don't put too much weight on that factor. This is most likely a matter of the Senior attempting to foster a dominate position with the pup. To solve the problem....YOU..the adult humans in the family need to secure YOUR position as the ultimate Pack leader of the family. This is best done by offering BOTH FEMALES some FUN, HAPPY, POSITIVE REINFORCEMENT TRAINING TIME...EACH DAY! You don't want to get angry or yell a lot at your older girl, because all that does is PROVE TO HER...that the puppy IS a horrible idea... Instead you work on all of your dog's good behaviors, Come, Sit, Lay down, Back up, Come up, Get down, Go to..your bed, the crate, the door...Find your...Ball, toy and you REWARD with really good treats and PRAISE! Work them separately because this is special time not only for training, but for bonding with each of them as their loving, fun, but clear LEADER. Remember a TRUE ALPHA reigns with BENEVOLENCE! This doesn't mean you are weak and wishywashy, you are strong, and clear in your commands and in helping each dog to understand what you want...but the having fun part is what helps to addict the dog to listening and learning what we want...practice each day maybe 30 minutes for the adult girl and 20 or less for the pup..turns the fun games into GOOD HABITS. What training does, is to give us ACCEPTABLE BEHAVIORS that we can ASK FOR and reward for, when our dogs are about to make a POOR or UNACCEPTABLE (to us) decision on their behavior! Plus, when the older girl starts to do something you do not want, you give her something else to do, and reward her for doing it correctly, which then starts to make her realize that when that pup is around...SHE...(THE ADULT) GETS MORE ATTENTION, PRAISE, TREATS...FOR NOT BOTHERING THE PUPPY! It starts to click...Pup in room, I leave pup alone, I SCORE! Which helps to cultivate the concept (and dogs are NOT great thinkers...they lack the mental agility of humans in the pondering department!) that this pup isn't all bad...I gets more attention and rewards when that pup is around. Training will also help develop a better self-esteem for both girls...success builds confidence in dogs too. The more relaxed and confident a dog becomes, they less they are compelled to ACT OUT aggressively to cope with their stress. Your Adult bitch IS stressed right now, and is acting out against what causes her to feel stressed! Dogs love routine. When normal routines change, they can easily become stressed, which can lead to potty accidents, and other behaviors we don't appreciate them doing. It is up to US to give them acceptable behaviors to do, when they are stressed, and to support them being able to do the good behaviors. One little trick you can use is at feeding time...when both are eating...Both of them need to be sitting politely and waiting for YOUR SIGNAL before they dive into their food bowls! Also, which ever one you feed first....(to the dogs) that is the one you are "supporting" as being the alpha bitch in your family... When we had to have our Sr. Bitch put down, my girl Hannah and our male, Patch, started having little scuffles for "who's in charge now?" I put a quick end to that, by supporting Patch at meal times. They both must sit quietly and politely, but Patch (little old man and smaller than Hannah) gets fed first! My situation was slightly different than yours because I was dealing with a male and female, and with dogs...FEMALES kinda rule over all in most ways...even though ours are also spayed and neutered...in dog's brain....females DECIDE what males get to breed to them! Even though Hannah is about the least sexually thinking female I've ever known...and Patch frankly doesn't want to hump her for sex or dominance....feeding him first, indicated to Hannah, that she needs to LIGHTEN UP on the old boy and stop trying to boss him around so much. You just want to remember, dogs do not think like humans do. Their priorities are NOT the same as ours, they do not think about HOW THEY FEEL...They ACT and REACT to situations they are presented with, according to their temperament, and training, needs and instinct. Training will give you a BIG BAG of acceptable behaviors to call upon, when they are about to ACT in a way you do not want! Let me know if you have any questions! Hannah my Search and Rescue dog and Patch taking turns at dinner with a Free Deer neck from a kind Freecycle hunter (I feed Species Appropriate Raw) The two of them last Christmas, listing to The Night Before Christmas, Patch likes to cuddle, and this night Hannah obliged him! Hannah working a tracking trail.