The tweet has gone viral in a matter of hours, with over 27,000 likes and over 4700 retweets.
Over 200 people have also commented on the post, with one stating "this is by far the most heartwarming thing I have seen all day".
Another wrote "this has cheered me up inordinately", while one person said it was "wonderful how a friend shows up just in time".
The post creator said the animals live on a small 20-acre farm near Nelson and are in no danger as "no one on the farm is eaten".
"I have seen them a couple of times when I have been visiting and thought it was a cute Sunday night story to tell to my 300 odd followers, but it has gone a little crazy," she said.
"It appealed to a lot of people. I wasn't expecting that. I thought it might get something like 20 Likes.
"I think it is just a heart-warming story."
She said a farmer told her that older sheep were sometimes bullied out of the pack because of their age.
"He is a very old sheep is McDuff," she said.
"Since he is a purebred Texel ram I can tell you precisely that he will be 11 on Tuesday of next week. That is pretty old for a ram!"
She said her brother was a "great farmer who really looks after his animals".
She said Pig was an orphaned wild pig when she decided to join the farmlet.
"It arrived of its own accord and settled down and they have been great friends ever since. I have seen them many times eating together, they are always together."
