How to reproduce:
1. Open the attached "Sibling" project
2. Open the "SampelScene" Scene
3. In the Hierarchy select the "0" GameObject which is a child of the "GameObject" GameObject
4. Delete the selected GameObject

Expected results: In the Console, the "Sibling Index During Destroy: 0" message is printed
Actual results: In the Console, the "Sibling Index During Destroy: 3" message is printed

Reproducible with: 2018.4.27f1, 2019.4.11f1, 2020.1.6f1, 2020.2.0b4

Notes:
- When deleting the child GameObject from Hierarchy it will print out the number of a sibling GameObjects - 1 as an index instead of where that GameObject sits in the sibling Hierarchy.
- When the OnDestroy() function is called not when deleting a GameObject through the Hierarchy, the GetSiblingIndex() will return the correct value