"it’s almost like she won’t change because she doesn’t want to admit she was wrong about the current EA."
Statistically speaking, your sister stands a far lower chance of selling the property if she moves it to a 2nd agent. So, although she might be doing out of stubbornness, she is actually doing the right thing.
"On average 71% of properties will eventually sell but with a second agent that drops to about 50%"
TwentyEA are a firm who gather market data for estate agents, but I've heard similar statistics spouted time and time again by less invested sources.
https://news.twentyea.co.uk/blog/do-over-fifty-percent-of-homes-sell-with-a-second-agent
"The tenants are moving out - after their offer was rejected, they bought another property and will be moving into that soon. At least that’s something."
I would strongly, strongly recommend that she only puts the house on the market (again?) AFTER they have physically left, because:
a) They're divorcing, people know dividing assets is messy and may avoid
b) Idk about Ireland, but in England, it can take a year or more to evict tenants, so if their house purchase falls through, it will hold the whole process up, her buyers will be disappointed and keeping an eye on the market for something better to come up
c) People who are living in terrible rentals (I was one til recently) will make a beeline for vacant properties, it will be far, far more attractive to a bigger pool of people and
d) If any minor DIY things need doing, she can do them without interruption and finally,
e) The tenants could badmouth either your sister OR the house; god, I'd need a full two hours to tell you all the things that were wrong with my last rental which probably needs around 150k spending on it to bring it up to working/properly habitable
Your sister needs to push the EA on a regular basis to find out what they're doing to progress getting feet through the door. Statistically speaking, she SHOULD stick with that agent. Over on Property/DIY, where I spend most of my time, the overwhelming opinion of posters who are asking about properties being moved to a 2nd agent is 'what's wrong with the house.'
Try not to get too tied up in this, it's stressful, your sister is an adult, she has to be responsible for her own decisions and actions.
But if she refuses to pay her share of the mortgage she will get a CCJ and get blacklisted by every mortgage lender, credit card company and buy now pay later firm out there. She won't even be able to get a mobile phone contract. That's the thing I would stress if she were my sister. 'I know you want your own way but you will royally eff your credit rating up for at least 7 years, so just don't do it.'