We perform a linear analysis of the stability of isothermal, rotating, magnetic, self-gravitating sheets that are weakly ionized. The magnetic field and rotation axis are perpendicular to the sheet, and we include a self-consistent treatment of thermal pressure, gravitational, rotational and magnetic (pressure and tension) forces together with Ohmic dissipation and ambipolar diffusion. We focus on two nonideal magnetohydrodynamic (MHD) effects (Ohmic dissipation and ambipolar diffusion) that are treated together for their influence on the properties of gravitational instability for a rotating sheet-like cloud or disk. Our results show that there is always a preferred lengthscale and associated minimum timescale for gravitational instability. We investigate their dependence on important dimensionless free parameters of the problem: the initial normalized mass-to-magnetic-flux ratio μ0, the rotational Toomre parameter Q, the dimensionless Ohmic diffusivity ηOD,0, and the dimensionless neutral-ion collision time τni,0 that is a measure of the ambipolar diffusivity. One consequence of ηOD,0 is that there is a maximum preferred lengthscale of instability that occurs in the transcritical (μ0 ≳ 1) regime, qualitatively similar to the effect of τni,0, but with quantitative differences. The addition of rotation leads to a generalized Toomre criterion (that includes a magnetic dependence) and modified lengthscales and timescales for collapse. We apply our results to protostellar disk properties in the early embedded phase and find that the preferred scale of instability can significantly exceed the thermal (Jeans) scale.