diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2dc6e3e7449..02121ff95b0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -908,26 +908,35 @@ if(CONVERT_CUDA_TO_HIP)
   set(USE_GPU_BRIDGE OFF)
 endif()
 
+# NOTE: Inline sibling search: duplicated from opm-common/cmake/Modules/OpmSiblingSearch.cmake
+#   because we need to locate opm-common before find_package(opm-common) makes that module available.
+# NOTE: If you modify this logic, you must also modify the logic in opm-grid/CMakeLists.txt and in
+#   opm-common/cmake/Modules/OpmSiblingSearch.cmake.
+# NOTE: See description of logic in opm-common/cmake/Modules/OpmSiblingSearch.cmake for more details.
 if(SIBLING_SEARCH AND NOT opm-common_DIR)
-  # guess the sibling dir
-  get_filename_component(_leaf_dir_name ${PROJECT_BINARY_DIR} NAME)
-  get_filename_component(_parent_full_dir ${PROJECT_BINARY_DIR} DIRECTORY)
-  get_filename_component(_parent_dir_name ${_parent_full_dir} NAME)
-  #Try if <module-name>/<build-dir> is used
-  get_filename_component(_modules_dir ${_parent_full_dir} DIRECTORY)
-  if(IS_DIRECTORY ${_modules_dir}/opm-common/${_leaf_dir_name})
-    set(opm-common_DIR ${_modules_dir}/opm-common/${_leaf_dir_name})
-    set(opm-grid_DIR ${_modules_dir}/opm-grid/${_leaf_dir_name})
+  # Strategy 1: Compute relative path from source to build dir, apply to sibling.
+  # Handles both flat (repo/build) and nested (repo/builds/debug) layouts.
+  get_filename_component(_source_parent "${PROJECT_SOURCE_DIR}" DIRECTORY)
+  file(RELATIVE_PATH _build_rel "${PROJECT_SOURCE_DIR}" "${PROJECT_BINARY_DIR}")
+  set(_sibling_common "${_source_parent}/opm-common/${_build_rel}")
+  set(_sibling_grid "${_source_parent}/opm-grid/${_build_rel}")
+  if(NOT _build_rel MATCHES "^\\.\\." AND IS_DIRECTORY "${_sibling_common}")
+    set(opm-common_DIR "${_sibling_common}")
+    if(IS_DIRECTORY "${_sibling_grid}")
+      set(opm-grid_DIR "${_sibling_grid}")
+    endif()
   else()
-    string(REPLACE ${PROJECT_NAME} opm-common _opm_common_leaf ${_leaf_dir_name})
+    # Fallback strategies for out-of-source or non-standard layouts
+    get_filename_component(_leaf_dir_name "${PROJECT_BINARY_DIR}" NAME)
+    get_filename_component(_parent_full_dir "${PROJECT_BINARY_DIR}" DIRECTORY)
+    string(REPLACE "${PROJECT_NAME}" opm-common _opm_common_leaf "${_leaf_dir_name}")
     if(NOT _leaf_dir_name STREQUAL _opm_common_leaf
-        AND IS_DIRECTORY ${_parent_full_dir}/${_opm_common_leaf})
-      # We are using build directories named <prefix><module-name><postfix>
-      set(opm-common_DIR ${_parent_full_dir}/${_opm_common_leaf})
-      string(REPLACE ${PROJECT_NAME} opm-grid _opm_grid_leaf ${_leaf_dir_name})
-      set(opm-grid_DIR ${_parent_full_dir}/${_opm_grid_leaf})
-    elseif(IS_DIRECTORY ${_parent_full_dir}/opm-common)
-      # All modules are in a common build dir
+        AND IS_DIRECTORY "${_parent_full_dir}/${_opm_common_leaf}")
+      set(opm-common_DIR "${_parent_full_dir}/${_opm_common_leaf}")
+      string(REPLACE "${PROJECT_NAME}" opm-grid _opm_grid_leaf "${_leaf_dir_name}")
+      set(opm-grid_DIR "${_parent_full_dir}/${_opm_grid_leaf}")
+    elseif(IS_DIRECTORY "${_parent_full_dir}/opm-common" AND
+        EXISTS "${_parent_full_dir}/opm-common/CMakeCache.txt")
       set(opm-common_DIR "${_parent_full_dir}/opm-common")
       set(opm-grid_DIR "${_parent_full_dir}/opm-grid")
     endif()