You just need to tell them.
They... have a way of life you do not agree with nor want as an influence in your life, nor do you want them to control you... and their "better than thou" attitude toward you and 'guilt' you and use you as a surrogate 'family.'
Them wanting to see you/your son is normal... though their way of doing it and their behavior is not good.
Your Mom does not have her Husband anymore and your sister does not have her Dad anymore... and all they have left is you and your son.
So, they may be clingy. But along with that, they have taken the stance that they have to 'control' you. Which is not good for you/ your son. Which thankfully, you are cognizant about..... and you are trying to live your life. Positively.
The only way to do this, is to tell them. But it will cause conflict. Because, people naturally do not take being 'rejected' very well.
They probably also feel "better than thou" because, your Sister moved back home to care for your Mom after your Dad died, and so they formed a co-dependent relationship and maybe feel that since you did not help with "Mom" after Dad died..... that you are not as thoughtful... although you are just more independent AND could manage your life just fine, on your own.
People do not 'have to' move back home, in order to get their lives back together, as your Mom and sister say.
They do not understand that. But since they are that way, they feel you are not as competent....
They seem to have a co-dependent type personality and are controlling. They don't 'see' that other people live differently and have different values. Thus, they act 'better than thou." It comes from ignorance and an inability to see other points of view.
It will be hard to 'satisfy' them... while also keeping your distance from their toxicity. Upon your life. So... that is the conundrum.
Perhaps... set up a "schedule" of visits with them AHEAD of time... so that visits is by YOUR choice, not theirs.
Say, only for your son's Birthday or something, or 1 Holiday.
Tell them you are very busy... and weekends are not suitable for them to just pop in. AND you also have a significant other and there is his schedule too and you/your son's/your Partner's lives too.
Have they ever met your Partner???? If this relationship is serious.. then, you have to lay the groundwork for implementing a "habit" of him being IN your life, in conjunction with their perceptions of it.... and make him a SOLID part of your life and your son's. So that, outwardly, to them... they will hopefully REALIZE that they also have to 'respect' him... as part of the scenario. AND that, they have to also ask permission of things from you and him... before just inviting themselves, over to your home.
OR... as an alternative, tell them they CANNOT visit nor unannounced... and that YOU will visit them. Say once a year. If that is feasible for you and what you want.
The thing is, will you always not want to see them, period? Or, will you be able to tolerate them, for once a year visits???? At your choice of timing??? Or, if you prefer not to see them, ever, ever again... then you need to realize that.
If not, they will always suggest coming to visit you.
You said you do not want them to visit.
So what does that mean exactly? Not visiting EVER, forever??? Or just sometimes???
...OR, ONLY UNLESS they 'behave' according to how you want them to be? If so, then that will probably never happen. People cannot change sometimes, or will not. And so, you either have to extract them from your life... or accept that they are difficult. AND, live your life anyway. Or you TELL them... you will not stand for their behavior and they have to, IF they want to visit you/your son... to go by YOUR 'rules.'
You need to tell them as well, that they CANNOT just drop in. And that, THAT is rude and disrespectful.
They probably are trying to infiltrate you/your life... because they have none of their own, and your Dad is no longer alive. And in conjunction with that, you have always been independent away from them... and they feel an incongruous 'need' to make you, into what they think you should be.
all the best,
Susan