In this paper, we investigate whether Fodor's (1998) same-size sister constraint (SSSC) influences the attachment preferences of English speakers when they parse ambiguous sentences involving relative clauses (RC), such as Someone shot the servant of the actress who was on the balcony. We look at how the SSSC interacts with prosodic boundaries. We predicted stronger attachment preferences when the position of a prosodic break (after NP1 or after NP2) and the SSSC (RC length matches NP1 or NP2) conspire to suggest the same attachment position. Fifty-six adult native speakers of English were tested on an auditory task, which manipulated position of the prosodic break, RC length and the lengths of NP1 and NP2, in eight different conditions. Contrary to hypothesis, overall results did not show the expected interaction between prosodic breaks and the SSSC. Nevertheless, we did find evidence, in some conditions, that the SSSC plays a role in biasing interpretation in the predicted fashion.