Utilize an attorney to get this done properly. The fact that you have a surviving sibling whom you wish to exclude could open this to challenges if not done right. Make sure you've got an executor in place as well, who knows your wishes - you won't want that role defaulting to your sister either.

In terms of the Humane Society, most good-sized charities and non-profits will have specialists who work with what is called "Planned Giving." Those are the folks you'll want to work with (along with your attorney) to sort out the details of what will be most useful and beneficial to them. They'll have a process, and likely will have worked with similar donations in the past.