Facts
The dispute concerns the validity of a deed of gift concluded on 18 April 2014 between [B.] , born in 1925, and her nephew [A.] , relating to an apartment in a condominium (unit no. 50623) and parcel no. 3991 located in the municipality of [U.] . In 1999 the parties had divided the co-ownership arising from the family estate.
From April 2012 the donor stayed in a home, following a syncope and a hospitalisation. Several medical reports from 2012 and 2013 diagnosed mixed dementia, or probable Alzheimer's dementia, with impaired cognitive results (MMSE 20/30, then 21/30 in October 2012, then 18/30 in October 2014). Witnesses confirmed the deceased's disorientation and memory disorders, and the home's note of February 2014 reported increasing disorientation requiring assistance with care and dressing.
On 18 April 2014 the donation was executed before notary [O.] , who testified that she had not detected any impairment and had not been aware of the medical reports; she acknowledged that she would have required a medical certificate had she known of them. After the deed, [A.] took his aunt to a café, which seemed strange to his cousin [C.] . The deed's correspondence was received by [A.] and forwarded late to [I.] , who usually managed the donor's affairs. A curatorship of representation and administration was instituted in January 2015, and the donor died in February 2015.
By judgment of 14 March 2017, the judge IV of the district of Sion, seised by [D.] and [C.] (the other heirs having submitted in advance to the outcome of the proceedings), declared the donation void and ordered the restitution of the apartment and parcel to the hereditary community. By judgment of 6 May 2019, the Cantonal Court of Valais dismissed the appeal lodged by [A.] .
[A.] lodges a civil law appeal with the Federal Supreme Court, principally seeking dismissal of the action and validity of the donation, and in the alternative remittal of the case. He alleges arbitrariness in the establishment of the facts (art. 9 Cst.) and a violation of art. 16 CC. No observations were requested.
Considerations
The Federal Supreme Court recalls that the appeal is admissible, the complaints being raised in accordance with the principle of allegation (art. 42 and 106 para. 2 FSCA), and that the review is in principle limited to the complaints raised. It decides on the basis of the facts established, which it may only rectify if their establishment is arbitrary (art. 105 para. 2 and 97 para. 1 FSCA; art. 9 Cst.), the party having to demonstrate arbitrariness with precision.
As regards capacity of discernment, it recalls that the party invoking the ineffectiveness of a deed must prove the state of weakness within the meaning of art. 16 CC and the impairment of the ability to act reasonably, the standard of proof being lowered to the balance of probabilities for a deceased person. A person who is durably in a state of mental weakness is presumed to lack discernment in relation to the disputed deed; this presumption may be rebutted by proof of a lucid interval or by demonstrating that, having regard to the nature and importance of the act, the person was able to act reasonably. Findings on mental state are questions of fact, whereas the conclusion on capacity is a question of law and is reviewed freely.
On the complaint of arbitrariness, the Federal Supreme Court considers that the appellant merely offers an appellate-style argument. The diagnosis of dementia was made by several physicians (oncologist, treating physician, geriatricians), and the adjective probable relates only to Alzheimer's disease and not to the existence of dementia. The witnesses, including the appellant's wife and the former wife of an heir, were consistent. The notary's testimony…