I'd like to see an update from you about what you and and aren't willing to do here..
Several people have suggested exactly what I would recommend -- the guests stay in a hotel or don't come -- but I can't tell from your post if you are going to be willing to get that tough with this relative. Are you?
Also, if they are coming in "a few weeks" you are close to, or already past, the window of opportunity to contact this person and tell her (not ask, tell) that you are e-mailing her a list of local hotels with contact numbers. If you are going to consider telling her she can't stay with you, you need to do it now. As in, today. Or the resentment and anger she WILL show will be even worse.
Because...she is going to be very angry and will interpret this as your criticizing her son. And you have to be tough enough to take that. You ARE criticizing her son -- and he deserves it. So does she, frankly, because her lenient and defensive ways mean she's raising a kid who is going to be in a heap of constant trouble very soon; I pity his kindergarten teacher and classmates if mommy is this far gone on "boys will be boys" and "gentle reprimands" for the words and behaviors your describe.
If you do plan to say, "I would like to see you and Son, but I am asking you to stay in a hotel," you must be prepared in advance to tell her why. Script what you will say or type, very carefully. Be totally prepared to have her go ballistic on you. Be prepared to have her say, "Well, we won't come visit, and we never will again after this" or even to curse you out. You need to be aware that she may say just that, and this stand on your part may cost you this relationship with her. If she is important in your life now or is a close relative you will run into again and again and again over the years at family functions -- you may need to consider carefully whether you want to end this relationship right now (which sending them to a hotel could do) or end it when you give her son a time out.
I think I would still vote for telling her that since the kids are at such different ages and stages, it is best if they stay elsewhere so the visiting time can be less stressful. But if you are reluctant to do that, then you must plan a LOT for handling this child for these couple of nights.
Enlist your husband or significant other or whoever else is in your home. Keep the kids VERY occupied all the time -- limit or just ensure there is no down time where the two kids are expected to "just go play so the grown-ups can talk/make dinner/whatever" and have an adult with this boy at all times. It will be stressful and if you do this, I would be sure never to issue another invitation to stay with you afterward.
It's doable to have them in your home but you must, must, must be willing to tell her simply and with a smile at the very start, "Our house has our rules, and we do use time outs and removal to stop hitting and to nip certain language in the bud. If you're not willing for Billy to be put by me or Husband into time out, or to be removed to another room, then we understand that, but it would break our house rules for kids, and we'd need you and Billy to think hard about either following those rules or maybe staying in a hotel and meeting up at other places for your stay.
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