[Edit] High effort post warning: I did a dive into a few sources, into the economic perspectives for people with disabilities in the USSR, with a small final discussion on my part at the end, the discussion being divided into two comments. The first source is in Spanish (since I'm from Spain myself) but I translated it to English below.
I happened to stumble upon a pamphlet called "Lo que recibe el trabajador soviético además de su salario" (what the Soviet worker receives beside their salary), from 1959, written by A. Zveriev, the then minister of finance of the USSR. Please note that it uses contemporarily wrong-sounding words to refer to people with disabilities, such as "inválido" (which translates to something like non-valid, I've translated it to "disabled" in English for the text), which was the normal term back in the day in the Spanish-speaking world to refer to people with disabilities. Since then, the term first evolved to "minusválido" (less-valid), and contemporarily to "persona con discapacidad" (person with disability). In the section dedicated to social security, it briefly says:
"El problema de cómo asegurarse en la vejez, en caso de enfermedad y pérdida de la capacidad del trabajo, ha preocupado siempre y por doquier y sigue preocupando a millones de trabajadores. [...] El seguro social soviético abarca a todos los obreros y empleados [...]. Esta determinación de beneficiario no excluye del seguro ni a un solo obrero o empleado en caso de pérdida temporal de la capacidad del trabajo por enfermedad u otras causas. [...] Una peculiaridad del seguro social soviético es el alto nivel de asistencia que garantiza a los obreros y empleados y sus familias durante el período de enfermedad, embarazo y parto o pérdida de la capacidad de trabajo por otros motivos [...]
En caso de pérdida temporal de esta se pasan subsidios en la proporción del 50 al 90% del salario, según los años de servicio [...] Los subsidios por enfermedad o accidente se hacen efectivos desde el primer día de la baja hasta el total restablecimiento o declaración de inválido al paciente por una comisión de expertos médicos. En tal caso, éste pasa a la categoría de pensionado [...]
La suma total de subsidios por incapacidad temporal del trabajo en 1959 asciende a 11,000 millones de rublos, es decir, el quíntuple de la cantidad abonada en 1940. [...]
Las pensiones del Estado se adjudican por vejez e invalidez y en cáso de pérdida del cabeza de familia [...] Las pensiones por invalidez se estipulan según el grado de pérdida de la capacidad de trabajo, correspondiendo tarifas elevadas a los obreros y empleados que hubieran contraído la invalidez a causa de accidente de trabajo o enfermedad profesional. [...] para 1966 se prevé elevar [...] las [pensiones] de invalidez."
Translated to English:
"The problem of how to secure oneself in old age, in case of illness and loss of ability to work has always and everywhere concerned and continues to concern millions of workers. [...] Soviet social insurance covers all workers and employees [...]. This determination of beneficiary does not exclude a single worker or employee from insurance in case of temporary loss of ability to work due to illness or other causes. [...] A peculiarity of Soviet social insurance is the high level of assistance it guarantees to workers and employees and their families during the period of illness, pregnancy and childbirth or loss of ability to work for other reasons [...]
In case of temporary loss of ability to work, benefits are paid in the proportion of 50 to 90% of the salary, depending on the years of service [...] Benefits for illness or accidents are paid from the first day of sickness or accident until the patient is fully recovered or declared disabled by a commission of medical experts. In this case, they become a pensioner [...]
The total amount of benefits for temporary incapacity for work in 1959 amounts to 11 billion rubles, i.e. five times the amount paid in 1940. [...]
State pensions are awarded for old age and disability and in the event of the loss of the head of the family [...] Disability pensions are determined according to the degree of loss of working capacity, with higher rates for workers and employees who have become disabled as a result of an industrial accident or occupational disease. [...] for 1966 it is planned to raise [...] disability [pensions]."
So basically, it seems that people with disabilities that prevented them from working, were treated as pensioners, earning a pension proportional to how long they've worked and their salary during working years. Additionally, from the text:
"La administración de los seguros sociales está estructurada sobre amplios principios democráticos. La ejercen los sindicatos [...] lo cual garantiza el mejoramiento del servicio y el control de masas para que los fondos del Estado dirigidos a estos fines se utilicen convenientemente".
Or, in English:
"The administration of social security is structured on wide democratic principles. It's exerted by the unions [...], which guarantees the improvements of the service and control of masses so that the State funds allocated to these ends will be used conveniently".
From this, and the part before where it says that "a peculiarity is the high level of assistance it guarantees to workers and employees and their families", I deduce that by talking with the union representatives in case of a disabled family member as a worker, it would be possible to have access to some form of extra income, but I admit this is just my assumption. [1/2]