It's a tough call. Sometimes it's too soon, and sometimes just thinking of the deceased pet is so painful - but then if they see a new and vibrant animal, they see that it's not the same animal and it can brighten their outlook. They are old enough to understand that you aren't replacing a pet right away - that's a problem with little kids when one pet dies and another appears immediately. They think everything can be replaced. But at 11 and 13, it's different.
One thing we did when our dog died was to donate the usable items to the local humane society - bowl, leash, good toys, etc. You could collect those things from the dog (since you aren't considering another dog now), and take the girls to the Humane Society to donate. Take old towels and sheets too - they always need those for all the new animals coming in. While you're there, let the girls take a look in the kitty room and just volunteer 10 minutes of petting some animals who've been abandoned or given up for other reasons. See what their reaction is. This way, your visit to the Humane Society isn't for the purpose of getting a new pet, but rather to help your Butterscotch and Buster "live on" by helping other pets with no family.
And I commend you for considering another pet from an adoption agency. We bought our last dog as a puppy, but wound up getting a rescue 3 years ago from a local humane society/shelter. My husband was like your nieces - not ready - but I was. I saw a dog written up in the local paper (they profile the adoptable animals from the shelter) and this one appealed to me for several reasons. I asked my husband to go with me but said I wouldn't do anything if he really hated the dog. Long story short, she was perfect for us, and he was lukewarm for about 4 days until she snuggled up with him. Now they're inseparable and she really helped heal the hole in our hearts. There's something about an animal who really needs YOU that makes the decision easier. Maybe something like this will work out for your family. But if your nieces aren't ready, just going to donate supplies will be a good deed in and of itself. (Call ahead to see what the shelter really needs.)