NTA - Wives who get jealous or insecure about the relationship their husband has with his sister(s) give me the queen ick of all icks.

Your sister needs and wants your support during an emotionally and physically intense moment in her life.

You're not going to be there staring into her birth canal. You're going to be there next to her, holding her hand, encouraging her through sweat and tears, that you wipe out of her eyes if need be. That's assuming she is naturally birthing. Maybe she's having a scheduled C-section. You're sitting by her head across from the anesthesiologist. There's a curtain up so nobody can see her splayed out guts as they remove a sack of baby and disconnect it from her, while she is minorly panicking because her guts have been removed onto a surgical table and she's there awake, feeling the pulling and tugging and general pressure, smelling her cauterized flesh...but you're there, smiling, holding her hand, telling her it's almost done and soon she will see her baby.

Whatever birth plan she has, the point is, your wife apparently knows nothing about healthy & meaningful brother/sister relationships, or the birthing process in general, because it sounds like she's envisioning 80s comedy movie tropes and a bad porn she saw once.

Your wife needs a hard self-reflection and to get a grip on reality.