I donlt beleive you will be able to do this, I'm afraid. Her son may be able to do it, but I don't believe you can.
Under the new GDPR privacy rules, DNA details can only be posted by the DNA owner or their immediate family - parents and adult children - not siblings.
Her son may have to join wikitree as a family member and be linked to his mothers profile as her son - but his profile will be unlisted since he is still living. His mothers profile will be open since she has passed. The son can then add the details of her DNA tests and they will then be propagated throughout the tree - which takes up to 24 hours to occur.