MonoGame.Extended/Source/MonoGame.Extended/Content/ContentReaderExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework.Content;

namespace MonoGame.Extended.Content
{
    internal static class ContentReaderExtensions
    {
        public static string GetRelativeAssetPath(this ContentReader contentReader, string relativePath)
        {
            var assetName = contentReader.AssetName;
            var assetNodes = assetName.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);
            var relativeNodes = relativePath.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);
            var relativeIndex = assetNodes.Length - 1;
            var newPathNodes = new List<string>();

            foreach (var relativeNode in relativeNodes)
            {
                if (relativeNode == "..")
                    relativeIndex--;
                else
                    newPathNodes.Add(relativeNode);
            }

            var values = assetNodes
                .Take(relativeIndex)
                .Concat(newPathNodes)
                .ToArray();

            return string.Join("/", values);
        }
    }
}