Mailman mentioned to Mom's neighbor she isn't taking mail inside. Her sister called the police and they went in the house. She didn't come out but told them she didn't feel well, didn't want to see her sister and wanted them to leave her house. She won't answer any phone calls or emails. She was having groceries delivered but the neighbor hasn't seen the deliveries lately.
Adult Protective Services can do a welfare check but she will have to be worse than just somewhat depressed if she is competent. I agree you - and probably Sis too - both need to go in person. Maybe you can persuade her you love her and she needs to go with you and get a medical evaluation. "Doesn't feel well" could be ANYTHING.
You need to get to her and check things out and get her to a doc to see if she has dementia or something else going on. Meanwhile use this site to learn about dementia and be able to recognize symptoms when you see her.
It could be caused by a number of things, including depression. I would certainly not delay. Check her bills, as she could have life insurance, health insurance or utilities that are past due or canceled.