Thread: I'm the MOM!

It sounds like your son's not the only one who needs to listen to you. From a few weeks old, at least one member of your family has been undermining your parenting, and from your description it sounds like others do it too? That's totally unacceptable, blurring the lines of who is "in charge." You need to stop the undermining behavior from the adults in your life immediately. This might be best started out with a sit-down talk with them. Are they the kind of people who will take you seriously in that kind of context? (I'm not sure that the type of person who will sabotage your child's upbringing would listen to you as an adult.) If they won't listen and continue to interfere, you will have to remove them from influence, at least temporarily. When your family members tell your son to do the opposite of what you say, tell them to stop immediately or you will leave. If they do it again, you have to really leave. And repeat this at every gathering until they stop, or refuse to attend gatherings until family agrees to stop.
Don't let them tell you that you're overreacting. Your thread title screams out to me that you need to feel like you're the mother in this scenario, and nobody is allowing you to do it. This is your child who you birthed (or chose/adopted, depending on your specifics) and to whom you are responsible. It's up to you to take anybody who is toxic out of his life as long as they insist on continuing to be toxic. You might even try daily affirmations: look in the mirror and tell yourself that you're the mom in this house, nobody else can do your job, and that the job you do is spectacular.

Thanks for the advise, your suggestions make a lot of sense and I plan to try them next time.

I had moments like that when my child was first born. You are woman hear you ROAR. You can do this, after all like you said you're the MUM. Good luck and all the best with taking back ownership of your child.