Well incest bad also because it tends to ruin families regardless of gender. Most of the time it resolves around an older sibling taking advantage of a younger child and has the power dynamics of any other type of pedophillia

Even when it’s equitable and between adults people’s psychology treats sexual partners differently. Introducing sexual relationship dynamics : potential jealouslies, the desire of siblings to grow apart and the secrecy pressures of keeping their relationship secret can up the risks of siblings hating each other.