Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
Poor Ivy, this isn't good. Often Gi signs like this are from gut infection, pancreatitis, or something risky being eaten. Though I have to warn you that these situations can quickly take a toll on them at this age, so we need to be proactive. The chicken is less likely to blame if her sister isn't showing signs.
First, if you think she ate something risky, we want her seen now to be safe. Else we can try some home supportive care just now. For this, you can try treating with an OTC pet safe antacid. [ie Omeprazole (0.25 -0.5 mg per pound every 24 hours), Esomeprazole (0.25-0.75 mg per pound once daily)]. Whichever you choose, we’d give this 20 minutes before food to allow absorption. I would just note that if you do get this from the chemist and its behind the counter (not on a shelf you can access), you may need to tell them its for you as there is a odd situation we see where sometimes they want prescriptions for pet meds even though its over the counter for humans and the same medication. Of course, if you give this and she cannot keep it down due to nausea that is usually a red flag that we need to bypass her mouth with injectable anti-vomiting medication from her vet.
Afterwards, boiled chicken is fine to give if she can keep it down. Other bland protein options you can use with Ivy are boiled white fish, or meat baby food (as long as its garlic/onion free)). Or you could use an OTC vet diets like Hill’s I/D or Royal Canin’s sensitivity control or gastrointestinal diet too.
For her diarrhea, fiber (ie canned pumpkin) and OTC probiotics (ie Benebac, Fortiflora) can be added to meals to firm those loose stools quicker and support digestion. Also if she is very loose, we can also try a pet safe anti-diarrheal like OTC Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/kaolin-and-pectin-kapectolin-k-p)). This is carried by most pharmacies. Alternatively, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (all OTC at vets, pet stores, and even Amazon) would be another option. These also slow diarrhea and also have probiotics to support the gut.
Finally as dehydration is a risk at her age with these signs, we need monitor that. To check that she isn't dehydrated, make sure her gums are moist (not dry), that she doesn’t have sunken eyes or a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a good video HERE- (How to Check Your Pet's Hydration)). If you are seeing any signs of dehydration already, then that would be our cue to have her seen before this becomes an additional issue for her (especially as it is often dehydration that makes them feel unwell).
Overall, her signs do raise a few concerns and we do want to be proactive for Ivy. Therefore, we’d want to start supportive care now. Of course, if she cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above within a few hours; then we'd want to get the local vet involved (some do have office hours on Sundays, so you could ring and see if that is the case). They can assess hydration, rule out fever, make sure there is nothing in her stomach that shouldn't be there or any sinister viruses present. Based on their exam findings, the local vet can treat with injectable anti-vomiting medication +/- antibiotics and fluids to settle this for her.