Why has my dog started becoming aggresive towards her sister?

My parents got my brother and I two dogs when I was 3, they’re sisters from the same litter. They’re both 14 now, 15 in November this year and they’re Chihuahua, Pomeranian, Yorkie and Jack Russel mixes. I don’t want to hear about how that’s unethical, I understand it now, however I was 3 at the time. None of us understood how mix breeds could be unethical.

My dog, Minnie, has never really been all that friendly in general but she’s much more relaxed compared to her sister who was a very hyper dog in her younger age. She’s much more of a ‘you can touch me only if I come to you first’ type of dog, she always has to instigate the affection unless it’s me, I can touch her whenever. She’s recently began being quite aggressive towards her sister and towards new people, my mum said Minnie tried to bite her three times while she tried to get her into a room so someone can come look at a housing issue, Minnie has never been friendly towards feet and I think it’s because she’s so small. She’s never tried to attack me before, nor has she ever growled at me. She allows all of us to pick her food up even mid way through eating and we can touch her while she eats as well.

She’s always been quite protective, she attacks the hoover any time it comes near me. Sometimes when her sister is too hyper and comes running towards me, Minnie would rush towards her and growl. We once went to the park and I was feeding geese as Minnie was laying on my lap, just watching the geese and one of them bit me, I said ow and minnie went for the goose. She didn’t get it, but I made sure not to say ow when they would bite me and she didn’t try again. She does have a predator drive only for mice, rats and foxes, she never goes for birds. She’s been going at her sister like crazy, it’s constant and I’m worried for her sister and she’s aging more than Minnie is. Minnie has never done any damage to her and she always fights back but I’m really scared for the day that she’ll be injured.

Could this be my fault? Have I been rewarding possessive behaviour without realising it?