Could you enlist your siblings to help exercise them? Have someone take them for a good long walk/run mid-day, or throw a ball for them for a while, etc? You can even make your own sort of agility set-up, and maybe enlist a sibling or two to help with that, and that will get them more invested in the dogs' lives and give the dogs more fun times! Even training to do "tricks" on command is good mental stimulation, and anyone can do that!