If you want to join your husband, registered partner or family member up to 2nd degree in his/her self employed business, as a general rule you cannot be employed on a contract, but will have to register as autonomo instead, as ´autonomo colaborador´.
The Autonomo colaborador is not obliged to file regular IVA nor IRPF tax declarations, the Autonomo Titular should.
In the case that the Autonomo can prove the family members do not live with him/her and are not economically dependent on him/her, as an exception, they can be contracted as employees. Very difficult in practice and the onus of proof that the relationship is indeed that of employer-employee, if the seguridad social questions the contract, is on the Autonomo,
Only exception are children under the age of 30. They can be employed on a contract as employees, but they will not build up any unemployment benefits, so won´t be able to claim those when their contract ends either.
More info
Under certain circumstances, for the Autonomo Colaborador there are discounts during the first 24 months (50% of the regular contributions for the first 18, 25% for the next 6), like there are for newly registered regular Autonomos with the Tarifa Plana. Please enquire with your gestor about the specifics.