My sister's horse is suddenly losing weight. Since my horse now lives with hers I am concerned that there might be parasites involved.

She doesn't want to call the vet, and instead wants to just up her feed. Should I more strongly suggest that she follow through with this?

I would call the vet. Parasites can be no joke if not caught early. I have lost animals due to parasites and was not even aware of them until it was too late. Since that time I have learned to be more observant of certain signs especially in winter when the thicker coat might conceal initial weight loss. I've also learned administer some basic medicines. If I suspect something is well under way like your friends horse I wouldn't mess with it for a minute myself. I'd get expert advice and or help.

You definitely need to get the vet involved. Early detection is key in making sure animals live through a parasite and remind your sister how bad she would feel if she not only killed her own horse but yours as well.

She is now responsible for two, not one horse. She needs to be made aware of the potential risks, and the fact that if your horse has lived through the same temperatures and conditions and not lost weight in the same manner than yes, there is likely to be something wrong. Even if it's not parasites, it needs checking out.

I would call the vet. There has got to be a reason the horse is losing weight. If it's noticable it's become a danger. Giving extra feed is not going to fix it.