Physically capable? Sure, I've seen a Boston terrier who had puppies that looked like the father was American bulldog. They were as big as she was by 12 weeks. Might require an expensive C-section, but possible.
Advisable? No. What is the purpose of the breeding? Have the parents been objectively evaluated for temperament and health? Been genetically tested for common health problems between the breeds? Is your sister prepared to be responsible for the puppies for the rest of their lives? Will she require any puppy buyers to sign a contract that they will spay/neuter their puppies and return them to her if ever they can't keep them?
If there is a purpose, she can answer yes to all the other questions, and has a few thousand dollars in the bank for vet bills, I think she is responsible enough to go through with it. Of course, there is still the potential for heartbreak as mentioned above. I have nothing against breeding if it is done well. Irresponsibly bred dogs fill shelters.
Here is the website of a breeder that does things the right way. She is a show breeder but that doesn't matter. She is responsible. https://www.knockoutchihuahuas.com/