This paper proposes two quadrature amplitude modulation (QAM) constellation integration schemes for recon figurable intelligent surface (RIS)-enabled symbiotic backscatter systems termed Schemes I and II, where the primary and backscatter signals merge into a standard 16-QAM constellation by offsetting the amplitude-phase coupling effects of a practically measured RIS, Specifically, the primary and backscatter signals adopt: (a) a binary phase-shift keying and a symbol subset of eight symbols selected from a 16-QAM constellation for Scheme I; (b) a quadrature phase-shift keying and a symbol subset of four symbols selected from a 16-QAM constellation for Scheme II. On this basis, we design the corresponding detector and derive closed-form expressions of the symbol error rate (SER) and throughput of the primary signal, the backscatter signal, and the whole system for the underlying schemes. Our theoretical and numerical results show that the SER and throughput of Scheme II outperform those of Scheme I at low and medium transmit signal-to-noise-ratios (SNRs), and tend to be the same at high transmit SNR.