I'd say cedar or cypress would do just fine. Even white oak would probably last a decent while. With the legs, the rot is most likely to happen at the top and bottom (exposed end grain). So having it on something that will drain away water (large gravel?) and protecting the top from having water sit on it would be very helpful.
The permie formerly known as "Mike Jay"
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"
The best raised beds I have ever seen were made of recycled guard rails from off freeways! The heavy wavy metal dove tailed nicely to stack and is strong enough to withstand outward pressure. For sure this isn't an option for most of us, but liked how it functions, even to preventing critters like squirrel from climbing up and in.
I have dry stacked rock- which is plentiful on our property - for making our raised beds. Using as natural as possible materials that are immediately at hand is always a good idea.
There is a minor draw back of raised beds that most don't anticipate - no 'toe space' (like on kitchen cabinets) where one ends up reaching over and can strain the lower back. And don't make tthe beds too wide so that your aunt has to stretch over to reach what she wants to plant/tend.