How Should I Handle This Situation With My Sister About Our Dogs?

I think you need to give her Todd and keep Mimi. Will your parents let you get another dog? Dogs are pack animals and I've always had 2 at least. Of course, you are the pack leader, so Todd's absence doesn't mean the "pack" ceases to exist. I just think Mimi would be happy with another dog to play with and it is good exercize for Mimi. There are a lot of unwanted pets that are very good dogs who need a home. I'm sure you understand your sister misses her dog, too. I have a Corgi and an "American dog of unknown ethnic origin." Mimi will probably become the dominant dog if you bring a puppy into the home. If you bring a full grown dog into the home, they will sort out their roles in the pack. I affectionately refer to my dogs as "the freeloaders."