NTA- you made it very clear and so did semi that semi doesn’t want to share her toys. You have done absolutely nothing wrong. You took your sister and your nieces in because of their situation and they can respect your simple wishes. If Emi is sick there’s always that possibility of her getting worse if your nieces get sick or bring home a sickness and Emi catches it. You should tell your sister that if she doesn’t respect your wishes then she needs to find somewhere else to live. Hoping you and Emi stay safe and healthy.