Whether it's justified or not is not really a question that can be answered objectively, but it's impossible for Speaker Pelosi to force Republicans to participate if they don't want to.
Initially, Congressional Democrats pushed to create an independent, bipartisan commission (modeled on the 9/11 Commission) to investigate. Such a commission would have been equally divided with 5 Democrats and 5 Republicans, and both parties would have had veto power over any subpoenas and the final report.
The proposed commission was modeled on the one established to investigate the 9/11 terror attacks, with 10 commissioners — five Democrats and five Republicans — who would have subpoena powers. A Democratic chair and Republican vice chair would have had to approve all subpoenas with a final report due at the end of the year.
However, Senate Republicans filibustered it, killing the legislation 54-35:
Bipartisan legislation to establish an independent commission to investigate the Jan. 6 attack on the U.S. Capitol has failed in the Senate, as Republicans staged their first filibuster since President Biden took office to block the plan.
So, the initial Democratic plan was to have an independent, bipartisan committee with equal power between the 2 parties. It was the Republicans who rejected that plan in favor of a House Select committee with more Democratic members.
With action in the Senate blocked, the only option for Speaker Pelosi is to create a House Select committee. The members of a select committee are selected (hence the name) by the Speaker and the party makeup is fully up to the Speaker, though they almost always accept the recommendations of the minority party.
In 2014, while the Republicans were in control of the House, Speaker Boehner created a select committee to investigate the terrorist attacks in Benghazi. That committee, like the current one, had an uneven (7-5) split between majority and minority parties and Speaker Boehner accepted Minority Leader Pelosi's recommendations for the 5 Democrats on the panel.
For the January 6th Select Committee, Speaker Pelosi also designed the committee with a 8-5 split and allowed Minority Leader McCarthy to pick 5 Republicans. She chose to veto 2 of his picks, a move she admitted was unprecedented, because of their public opposition to the committee and open intention to disrupt the investigation, but McCarthy could have submitted replacement candidates. Instead, he chose to pull all his recommendations in protest, which gave full power to name members to Speaker Pelosi.
Speaker Pelosi cannot force Republicans to participate if they don't want to, and, frankly, the fact that she appointed 2 Republicans, when Minority Leader McCarthy recommended 0 Republicans, is better than could be expected. At every stage of the process, Democrats attempted to create a non-partisan committee where Republicans would have power, and at every stage Republicans rejected that power in favor of a more partisan panel which would give them more power to complain about "bias".