Weimaraners only come in one colour, grey. Although blues do exist and are bred in the States they are not recognised and no reputable breeder that belonged to any breed club in the world would deliberately breed from dogs that did not fit the standard.
Weimaraners are no more and no less prone to "sensitive stomachs" than any other breed.
If your sister wishes to have the KC registration papers signed over to herself it is necessary to get them signed by the previous owner.
Both the stud dogs mentioned are well known and are in fact only Sh Ch not full Ch and Finn has just died. They were bred by Stephen and Patsy Hollings in Leeds and I am sure they will only be too please to offer assistance.
Quite frankly one would have thought that before obtaining a dog of any breed that one would have done a little research prior to acquisition

An 18 month old entire male weimaraner is not for the faint hearted; they are highly intelligent dogs that require a great deal of training and exercise. They do not mature until 3 and in the wrong hands can become a complete menace. They are a curious mixture of the sensitive and stubborn and do not react kindly to confrontational methods. They are certainly not a breed I would recommend for a first time dog owner, particularly males who can be quite assertive with other males and in fact with everything else if allowed to. They are fine with children if they are good with them!
A Weimaraner requires around 2 hours of off lead running per day, a 15 minute pootle round the park will only result in a dog that will take out its frustrations on the furniture, garden and/or become very noisy.
If you need any further help I would recommend that your sister gets in touch with the Weimaraner Club of Great Britain
http://www.weimaranerclubofgreatbritain.org.uk/html/rescue.html who will be able to offer assistance if required.
HTH