From https://answers.unity.com/questions/425012/get-the-instance-the-serializedproperty-belongs-to.html?childToView=425602#answer-425602 with support for SerializedDictionary

SerializedPropertyGetParent.cs

public static object GetParent(this SerializedProperty prop)
{
    var path = prop.propertyPath.Replace(".Array.data[", "[");
    object obj = prop.serializedObject.targetObject;
    var elements = path.Split('.');
    foreach (var element in elements.Take(elements.Length - 1))
    {
        if (element.Contains("["))
        {
            var elementName = element.Substring(0, element.IndexOf("["));
            var index = Convert.ToInt32(element.Substring(element.IndexOf("[")).Replace("[", "").Replace("]", ""));
            obj = GetValueAt(obj, elementName, index);
        }
        else
        {
            obj = GetValue(obj, element);
        }
    }

    return obj;

    IEnumerable<Type> GetHierarchyTypes(Type sourceType)
    {
        yield return sourceType;
        while (sourceType.BaseType != null)
        {
            yield return sourceType.BaseType;
            sourceType = sourceType.BaseType;
        }
    }

    object GetValue(object source, string name)
    {
        if (source == null)
            return null;

        foreach (var type in GetHierarchyTypes(source.GetType()))
        {
            var f = type.GetField(name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance);
            if (f != null)
                return f.GetValue(source);

            var p = type.GetProperty(name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
            if (p != null)
                return p.GetValue(source, null);
        }
        return null;
    }

    object GetValueAt(object source, string name, int index)
    {
        var enumerable = GetValue(source, name) as IEnumerable;
        var enm = enumerable.GetEnumerator();
        while (index-- >= 0)
            enm.MoveNext();
        return enm.Current;
    }
}

In my case it was not working for SerializedDictionary, my custom property drawer was associated to a property inside a dictionary value and the dictionary values are serialized in a private field of the "SerializableDictionaryBase" base class.

So I browse through all base types to look for the member and it works.