Inside Charles Spencer and Wife Karen's 'Unbelievably Beautiful' Renovation Plans for Princess Diana's Grave

The tiny island where Princess Diana was laid to rest – on the grounds of Althorp, the 550-acre English estate where the late royal spent part of her childhood – is in the midst of "beautiful" renovation, her brother Earl Charles Spencer tells PEOPLE in an exclusive interview in this week's issue.

"Before it was a sanctuary, very natural," he says of the project overseen by his wife, Countess Karen Spencer, whose nonprofit Whole Child International is focused on improving the lives of children living in Third World orphanages. "Now it will be a beautiful garden, unbelievably beautiful and appropriate."

Countess Spencer will soon be opening the 508-year-old estate, alongside her husband, in a fundraising effort for Whole Child – which focuses on strengthening the relationship between caregivers and children.

A handful of donors to the non-profit – which is preparing to launch programs in 365 facilities in El Salvador – will have the opportunity to experience a traditional English country weekend at Althorp. (Starting price: $25,000.)

Landscapers are currently in the midst of planting forget-me-nots and rhododendrons on the island. "They were Diana's favorite flower," says Charles, 52. "I still remember giving her some when we were children. When I was six, I gave her a white pot of blue forget-me-nots."

Rhododendrons were also chosen, says Charles, "because they were the flower at Sandringham," the estate where the siblings lived before moving to Althorp in 1975.

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The multi-million dollar renovation project will also include a physical memorial to Diana that workers are on the verge of finishing. "Before, we left the grave unmarked because the family knew where it was," explains Spencer. "But now we're going to have a memorial to Diana."

Remodeling work on the island – and gardens on the surrounding grounds – will be completed in August 2017, coinciding with the 20th anniversary of Diana's death.

Clearly, the Princess of Wales' presence still looms large at Althorp. The couple's youngest daughter, 3-year-old Charlotte Diana, says Karen, 44, enjoys rowboat rides to "visit Auntie Diana" on the island. "She feels very present," adds the countess. "For my husband, Diana was the person he was closest to his whole life, so it's a big void for him.

"My daughter has wonderful feelings because we often go to visit her on the island."