Polish United Workers' Party appeared in 1948 as a join of Polish Work Party and Polish Socialist Party. All members of both ancestors automatically became members of the Polish United Workers' Party. So, the reason of the personal terror couldn't be the struggle for individual members.
Communists simply wanted socialists leaders to agree with that join. Here the terror was useful. That terror for Socialist Party was named "The Cleaning". What is funny, they were not communists who did the dirty work, but the socialist themselves. Communists proposed to socialists the power, socialists fell apart in two factions, one for join with Peasant Party, and the other, under the lead of Cyrankiewicz, for join with Communists. Cyrankiewicz was paid by the post of prime minister and having the power, prosecuted his own ex-comrades. He became a very strict communist. Bierut got the power in 1952, much later.
It was the well known tactics of communists - salami tactics = old good "divide and conquer".
And that method of taking families for hostages is also old and well known tactics of communists. They used it in the civil war in Russia in 1918-21 years massively. All officers in Red Army had their families taken as hostages.
As for Russia, they had more fine tactics there: who did not help them was proclaimed for a non-working person and automatically didn't get any food. What meant a death, with families, too.