Firstly, I am somewhat puzzled as to how you can have an ancestor who died without an heir. I presume you mean that he had no descendants. Clearly he had an heir because you say that he left a will. An heir is defined as a person who inherits or is entitled by law or by the terms of a will to inherit the estate of another.An ancestor is a person from whom one is descended. However, taking your question to mean that you have someone in your family tree who had no descendants but who had an heir in the form of the children of his niece, then in order for those children to be entitled to his arms he would have had to have made specific provision in his will by way of a name and arms clause.

It may be possible to obtain a copy of the will to clarify if the arms were mentioned.

Regards,
Martin
http://cheshire-heraldry.org.uk/weblog