G.R. No. 196470 - Victoria vs. Pidlaoan

Petitioners Rosario Victoria and Elma Pidlaoan lived together since 1978 until Rosario left for Saudi Arabia.
In 1984, Elma Pidlaoan purchased a 201-sqm parcel of land in Lucena City, registered under Transfer Certificate of Title (TCT) No. T-50282 in her name alone.
After Rosario returned from Saudi Arabia, Rosario constructed a house on the lot, then left again.

In 1989, Elma allegedly mortgaged the property to Thi Hong Villanueva.
When the property was about to be foreclosed, Elma sought help from her sister-in-law, Eufemia Pidlaoan.
Eufemia contacted her daughter, respondent Normita Jacob Pidlaoan, who agreed to lend money to Elma to redeem the property.

Elma initially tried to sell the land but failed to find a buyer, then offered to sell it to Eufemia or Normita.
On March 21, 1993, Elma executed a deed of sale ("Panananto ng Pagkatanggap ng Kahustuhang Bayad") transferring the lot to Normita; it was signed by both parties and two witnesses but not notarized.
When proceeding with notarization, the notary advised that Elma instead execute a deed of donation to avoid capital gains tax.
On the next day, Elma executed a notarized deed of donation in favor of Normita.
Consequently, TCT No. T-50282 was cancelled and TCT No. T-70990 was issued with Normita as registered owner.

Normita paid the real property taxes but Elma continued occupying the house.
Rosario discovered the donation transaction after returning to the Philippines a year or two later.
In 1997, petitioners filed a complaint for reformation of contract, cancellation of TCT No. T-70990, and damages with a prayer for preliminary injunction against respondents Eufemia, Normita, and Herminigilda Pidlaoan.
Petitioners' claims:

Respondents admitted the deed of donation was simulated and the original transaction was a sale but denied any mortgage agreement.

The Regional Trial Court (RTC) ruled that Rosario and Elma were co-owners, and Elma donated only her half share to Normita.
Respondents appealed to the Court of Appeals (CA).
The CA reversed the RTC, ruling that Elma donated the entire property to Normita and that the deed of donation was not simulated.
The CA relied on the presumption of regularity of the notarized deed and found no conclusive proof to rebut it.
Petitioners moved for reconsideration, which the CA denied, prompting the petition to the Supreme Court.

Are petitioners Rosario and Elma co-owners of the lot?
Was the deed of donation executed by Elma in favor of Normita simulated?
What is the true nature of the transaction between Elma and Normita: sale, donation, or equitable mortgage?