I think the op has been a little remiss in not teaching her kid to
I think in my house it maybe works out more balance because everyone f needs, so when there is a conflict of needs, there needs to fair compromise, instead of
If the DS has never been taught that other family members also have needs that matter (eg. Nephews need for sleep), then that's more on the op then the young man, but at 25 he does need to learn.
I think in my house it maybe works out more balance because everyone has needs, so when those needs conflict, someone has to compromise and there's not a default 'more able to compromis' sibling that it alway falls to. For us, the 'rule' is that one person's right to 'be themselves' dose not extend to harming someone else, and if you're the one causing harm, the onus is on you to change.
Eg. My DD would like to authenticly express her displeasure by hitting me when I ask her to brush her teeth, but I'm not ok with being hit, so she makes do by eye rolling and yelling "fine!" (The onus is on her to control her hands)
My DS would like to 'be himself' by climbing onto furniture and jumping down (movement sensory seeking), but the constant crashing disturbs the neighbours and was causing sensory overload for DD, so he needs to do his jumping on a trampoline or gym ball. (Ie. Onus on him to use quiet jumping outlet, not on DD to adapt to sound)
In the op's case, the son likes to be himself by shouting when talking (maybe auditory sensory seeking?) But this is causing physical harm and emotional distress to his sister and nephew, who's sleep is being disrupted. It makes sense to me that the son should be the one to change, because he is the one who is harming others.
While I don't think the op should restrict his visits, she does need to make him understand that he can't keep shouting while the baby is sleeping. If she allows him to keep harming the baby, it will be impossible for the daughter to remain living with her, and a doubt her relationship with the daughter would survive her choosing the sons wants over her/her babies physical needs.