Inbreeding increases parent-offspring relatedness and commonly reduces offspring viability, potentially affecting selection on reproductive interactions involving relatives and associated parental investment. Separate bodies of theory predict selection for inbreeding versus inbreeding avoidance, and selection for optimal parental investment. We unify inbreeding and parental investment theory, demonstrating that these separate bodies can be interpreted as special cases within a broader inclusive fitness framework. We show that optimal investment per offspring increases when parents inbreed and hence produce inbred offspring with lower viability. Offspring viability is buffered by parental investment, but the total number of offspring that a parent produces is reduced due to an intrinsic trade-off between parental investment and offspring production. Optimal parental investment does not depend on whether a focal female is herself inbred. However, inbreeding causes optimal parental investment to increase even further given strict monogamy and associated biparental investment as opposed to female-only investment. Our model implies that understanding the evolutionary dynamics of inbreeding strategy, inbreeding depression, and parental investment requires joint measurement and understanding of the expression of each in relation to the other, and we demonstrate an intrinsic link between inbreeding and parental investment that may widely affect the evolution of behaviour and intra-familial conflict.