Hey OGTS, as a suggestion to your sister, she should get in contact with the beagle rescue she adopted from, as they may want Darwin to come back to them. Some rescues have policies regarding rehoming the animals, and most are willing to take back the animal if the adopter's circumstances change to where they can't keep the pet. That would likely be a good first step for her to take unless you're interested in adopting him yourself.

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"The worst sin towards our fellow creatures is not to hate them but to be indifferent to them: that’s the essence of inhumanity." -George Bernard Shaw