Oh, I seem to get the point. Thank you, Sharon!

Hi Irene,咱们/我们means we, 咱们的/我们的means our or ours. Sometimes in spoken Chinese, you can omit 的 but it is not grammatically correct in writing. It is true sometimes some people like to omit 的 when they speak in a very informal occasion. 咱们大哥，咱们弟弟is probably all right, but you are not supposed to say咱们爸，咱们妈，咱们宠物，咱们狗，咱们猫，you can say 咱爸，咱妈, 咱妹妹，咱哥，咱弟，but not 咱们弟弟们（咱们弟弟is probably understandable but still not right) . All these are all northern Chinese speaking. When you are practicing writing, you probably want to use 我们，我们的 instead. It would sound more understandable at this stage of practicing language. Plus, it's better to use 兄弟们instead of 弟弟们 here. Hope this helps.