Find parent SerializedProperty

SerializedPropertyExtensions.cs

using System.Linq;
using System.Text.RegularExpressions;
using UnityEditor;

namespace Monry
{
    public static class SerializedPropertyExtensions
    {
        public static SerializedProperty FindParentProperty(this SerializedProperty serializedProperty)
        {
            var propertyPaths = serializedProperty.propertyPath.Split('.');
            if (propertyPaths.Length <= 1)
            {
                return default;
            }

            var parentSerializedProperty = serializedProperty.serializedObject.FindProperty(propertyPaths.First());
            for (var index = 1; index < propertyPaths.Length - 1; index++)
            {
                if (propertyPaths[index] == "Array" && propertyPaths.Length > index + 1 && Regex.IsMatch(propertyPaths[index + 1], "^data\\[\\d+\\]$"))
                {
                    var match = Regex.Match(propertyPaths[index + 1], "^data\\[(\\d+)\\]$");
                    var arrayIndex = int.Parse(match.Groups[1].Value);
                    parentSerializedProperty = parentSerializedProperty.GetArrayElementAtIndex(arrayIndex);
                    index++;
                }
                else
                {
                    parentSerializedProperty = parentSerializedProperty.FindPropertyRelative(propertyPaths[index]);
                }
            }

            return parentSerializedProperty;
        }
    }
}

To get index in parent

public static int GetIndexInParent(this SerializedProperty serializedProperty)
{
    SerializedProperty parent = serializedProperty.FindParentProperty();
    if (parent != serializedProperty)
    {
        for (int i = 0; i < parent.arraySize; i++)
        {
            var prop = parent.GetArrayElementAtIndex(i);
            if (SerializedProperty.EqualContents(prop, serializedProperty))
                return i;
        }
    }
    return -1;
}