My sister has a cat, I go over to her house to take of it, when she needs it. But the thing I would like to know is he always meows. What could that mean?

He is saying he is happy to see you. Thank you for taking care of him.
Meowing usually means being content.

Sometimes I thought he could be lonely because she had another cat but it got that fiv

Well I have cats.
And when they meow they want food ,attention or even milk.

Our four cats meow to say hello and hope that we feed them soon. They make other sounds too, like little happy sounds or "what are you doing?" sounds or "leave me alone" sounds. Some cats 'talk' a lot and some don't have much to say--sort of like people.

Yup you are probably right, but he likes to follow me around the house. I think that is cute:p :p

Our cats have each attached themselves to one person in the family. My cat is the part-Abyssinian one named Thomas Jefferson. He talks a lot and likes to be with me, to watch everything I do and to lie on a newspaper or magazine or book when I try to read it. Maybe your sister's cat has chosen you to be a special friend.

Cats Meow, Dogs Bark, Birds whistle, Horses neigh.

That's just the way it is.

I just love yhis cat. He is funny. And black and white with green eyes. We think he is a Maine coon cat cause he has a bushy tail.

Cats are not really vocal creatures. Feral cats do not make much noise at all. Domesticated cats see us as the dominant cat in the house, and they learn to communicate with us by making cat noises. Meowing, chirping, meowing and purring at the same time.
The kitty could be trying to tell you something, just letting you know he is there, or just saying hey. All of my cats have a different way of talking to me, and they use different sounds depending on what they want. They are such wonderful curious creatures.

To be honest, it could mean anything. You have to match the body language too, and what it's doing. If it's rubbing against the walls when you walk in, it's like Hello.

If it continuously meows, I don't think that's too good, give it some attention.

Always pet it, and show love, feed it... with some treats... don't make it fat!. etc.etc