I have 2 older boys aged 6 & 4, so she's the third and I've not had so much time to linger over baths - lots of washes instead .
Nonetheless, she was ok with having baths until about a month ago when she saw her big brother pour water over his own head. He wasn't in the least bit distressed - he does it all the time - but she was, and I've been unable to bath her since. She even gets upset at the sight of the water in the sink I use to wash her.
She's quite a tough little thing in other ways and puts up with loads from her big bruvs with plenty of smiles.
Do you actually put her in the sink or use a face cloth? What about a tiny bit of water in a bowl and top and tail her for a bit. Let her see her siblings playing in the bath, but ensure theres no hint that she has to get in.