How do I discipline my 4 year old when there are people contradicting what i say?

My son's father and I recently seperated then decided to get back together. We haven't moved back in yet and once we move back in together with our son discipline will be much easier.
I'm currently living with my parents and everytime I try to discipline my child my mother steps in and tells him that everything is OK and pretty much undermines my authority. My dad is good about stepping back and letting me handle it. I think he thinks I have the right idea about it. I realize that my son needs uniformed discipline and it really drives me crazy that both my mother and sister undermine my authority.
Today, my son told me "no" when I asked him in shock what he said, he goes "i told you i said NO". Frustrated I told him to go to time out, once i finally got him downstairs and in the corner where his time out is my mom started playing with him, singing songs and so on. I told her to stop and she asked me why he was in time out. I told her and she shook her head and snapped at me. A few minutes later I call him to my side and ask him a series of questions "Do you know why you were in time out?", I like to make sure he knows why he's being punished and how he can avoid it next time (by listening). While I was talkng to him my sister walks up and asks him where his shoes are and tells him to put them on so they can go for a walk. Angrily I tell her to stop because she's just making it more difficult. I told her to let me discipline him in my way and she goes "Thats so wrong"
Anyway, I'm just wondering how I can deal with this when they're clearly undermining my authority even though I've spoken with both of them to stop.
Once his father and I move back in together I won't have a problem with the discipline. We always had an understanding that if one of us sent him to time out or his room that we have a good reason and we're the only one that can take him out. It actually works wonderfully.
So how do I deal with this complete lack of respect of my discipline techniques?

[Sandman2]

Tough situation here being in your parents house.  They have their own ideas on discipline.  What you need is some outside support.  I would suggest that you get   a copy of Lynn Clarks' book SOS: Help for Parents.  Read it, and then when your mother or sister disagree, pull out the book.  You also might ask them what they might do instead of what you are doing.  While consistency is very important, it never hurts to try new things.