vasya85,
I would recommend you first search of the survivors of former employees of the museum and they know if you remember, as was the case.
The museum could not "accept the gift of" painting from a private individual in the Soviet era. I know this from personal experience with a painting by JS Kopina.
The museum was obliged to buy them at least for some money that had been reporting in spending the money. Money from the Ministry and spend it on some of the museum workers without a command from above could not.
Therefore, they could buy a piece of work for a very low price, and the second part enter in the books as a "gift".
In any case, as mentioned above, the museum must remain some papers.
You must be "on-quiet" to meet with the chief curator and ask whether there are any documents on these pictures.
However, in turbulent times adjustment all the papers (or part thereof) could be eradicated in order to work have been orphaned - they were then easier to steal.
In general, this issue must be approached with extreme caution, "intelligence"
and then act according to circumstances.
I would have advised to "find" witnesses, friends of parents who would have confirmed that the paintings were put on deposit.