Hello southeastastra. So sorry to hear about your sister. Hopefully she is being thoroughly investigated for possible causes of this DVT. If she smokes she needs to stop. Smoking pushes the risks right up.
Warfarin interferes with the chain reaction known as the clotting cascade, which is the process that brings about clotting. People respond to warfarin differently and so the dose required will vary between individuals.
Lots of things can affect warfarin - including alcohol, dark green vegetables, some painkillers, including those bought over the counter, alcohol and some herbal remedies, in particular St John's wort and cranberry preparations, and multivitamin tablets, if they contain vitamin K.
However, as yomellamoHelly (sp?) mentioned earlier, the key thing is to remain consistant with diet and alcohol intake, though obviously, not to excess.
Your sister will be given an information pack containing advice about diet, unsuitable medications and general advice about staying well and safe. She will have a yellow book that she should keep with her, which will contain important medical information and a record of her INR (blood tests) results and warfarin dosing.
Some people on warfarin do take aspirin as well, but it has to be prescribed as part of specific management for their clotting disorder. Generally, people on warfarin should not take any medication without checking with a Health Practitioner TRAINED IN ANTICOAGULATION. Sorry about the capitals, but not all health professionals know everything about warfaring treatment.
Some people can eventually manage self testing, using a machine and test strips that are similar to those used by diabetics to check blood sugar levels. Otherwise she will either attend hospital, or in many areas either the GP or local pharmacy can do routine testing and dosing.
There is a very active and knowledgable patient support group - the name escapes me at the moment, but your sister's doctor will be able to put her in touch with them.
Sorry about the essay - I hope it answers some questions, and I hope your sister feels better soon and adapts to her treatment.
HTH