Change in dog behaviour

Question
My female beagle/dalmation/lab mix Princess is 3 years old. We got her from a shelter when she was 9 weeks old and at the time had a 9 year old female mixed breed, Sunny. They got on great, they really were best friends. 2 years ago we moved in with my mother-in-law to take care of her as she is disabled and she has a (now) 9 year old rottweiller/lab mix, Missy, the soppiest dog you could wish to meet. The 3 dogs got on fantastically! we were elated! Occasionally Sunny and Missy would get into a spat over a milkbone or something stupid but nothing resulting in injuries, just slobber covered dogs and the occasional nipped ear or lip. Sunny was definitely the 'alpha female' and she made sure everyone knew it!
We had to have our darling Sunny put to sleep a couple of weeks ago and we expected some grief and/or depression from the other dogs, especially Princess as she had been with her from such a young age. Princess was a little irritable with Missy but we found that if we let her sniff Sunny's collar she was ok. Finally we let Princess wear Sunny's collar (it was the only thing that seemed to 'pick her up' and Missy was just 'Missy'. Now Missy is a big dog, 9 years old as I said, has arthritis and does not do much all day. She was my father-in-law's dog before he passed away and she would just amble about the yard with him or lie at his feet in the house. She had been depressed over losing her master but when we moved in it seemed to brighten her up again.
Princess has suddenly started attacking Missy for no reason. We understood some of the initial aggressiveness as Missy has a bad habit of stealing Princess's dog biscuits - Princess never eats them right away - they get hidden behind sofa cushions or out in the yard but she goes back for them later. If I find them I normally put them up for her for later.
My husband reckons that Princess is just using her dog intuition that Missy is old and 'sick' and if they were in a pack in the wild would attempt to rid the pack of the older or sick dogs that hold them back or who can't hunt. She has become the alpha female.
Princess will be quite happy for a couple of days and not bother Missy at all, but then might try to go for her a couple of days in a row. Poor Missy cannot fight back or run away because of the arthritis and just lies there rolling around and crying. Princess never draws blood, just seems to slobber all over her but it is obviously frightening to Missy and she seems to be getting very depressed over this. She just lies in the corner near my husband, she goes out to pee etc in the yard maybe twice a day (not like her at all - she is the queen of pee'ers) she doesn't do the 'dog biscuit dance' which she would do as often as possible in the hope that you might forget she had a biscuit a few minutes ago.
Did I do the wrong thing with giving Princess the collar? Or is it just the alpha female concept?
I am so torn here as Princess is my baby but I am very upset that she is hurting such a dear old friend, who has NEVER hurt her. I am also worried that if I rehome Princess, Missy will get even more depressed and die of a broken heart. I should mention that even tho Missy is my 'in-laws' dog, due to my mother-in-law's (MIL for short) disability there wasn't much bonding there, but my MIL is in and out of hospital every few weeks and I wonder if that could have anything to do with it as well. Just a point to add or ponder really.
I don't want Princess to have to take pills to calm her down, she is usually such a happy, friendly dog. She loves our cats, loves our kids and has never hurt a fly (well she kinda had a love hate relationship with a muskrat who used to wander onto the back porch and eat the outside cat food until a few months back).
My usual vet is going thru some personal problems at the moment and I'm not keen on asking his advice and as I live in a small place I don't have a huge choice of vets.
I hope you can offer some advice. I have read thru other sites with advice but they don't help really as this is quite a unique situation.
Thank you!
Sarah

Answer
Hello Sarah,

What would help Missy most is treating her for her arthritis so she feels better.  Starting her on a joint supplement with glucosamine and MSM will help lubricate her joints and then asking your vet about anti-inflammatories such as Etogesic, Rimadyl, Metacam or Deramaxx to help ease the pain.  Maybe if she feels better, she won't be so irritable.

You can try taking the collar away from Princess, but I doubt that is what sparked the whole issue.  Your other dog may have been the mediator between the two and now that she is gone there isn't anyone to settle the issues.  You can try adding RESCUE REMEDY to their drinking water to see if that helps.  You may also want to consider talking to an animal communicator to see what is really the issue ... Anita Curtis is a good one... www.anitacurtis.com

Best Wishes!