Is it Islamically permissible to make a will bequeathing half a shop to each son or daughter, and is there a sin incurred by not relinquishing the inheritance in the shop and half of it in favor of the three sisters?

If your mother verbally bequeathed the shops, this is a bequest to an heir, which is not permissible unless all adult and sane heirs agree to it. If her bequest was made valid by registration for those to whom she registered, excluding the others, then the gift is valid only for those to whom it was registered, not for those who were not registered, and the share of the unregistered individuals is considered part of the inheritance. However, if she gifted the shops during her lifetime and before her death, the gift is established by possession and relinquishment to those to whom they were gifted, and all the shops become your property.

The matter should be referred to the judiciary to consider the details of the issue.