I think it will hurt them slightly and here's why. The Broncos and Lions have their core fan bases. Broncos around 20k - 25k, Lions around 10k - 15k. Then you have your RL fans that aren't Broncos fans and turn up usually to support the opposition team because they are a fan. This ranges from 5k to 10k depending on the team. For AFL this seems to be about 5k to 10k also (expat Victorians that want to watch their team when they are in town).
Then you have the battle ground fans. The 10k or so that are casual sports fans and want to attend the events or what is trendy at the time.Usually that is the Broncos but when the Lions are winning they enjoy support from this group. Some of these fans also just want to watch footy every weekend, not every second weekend. This is where the NRL could really hurt the Lions by putting a second team here.
Firstly, the new team will be shiny, new and trendy. Secondly, you have a second opportunity for non-Broncos fans to attend as an away supporter (so potentially drawing them to two games a year not just one). Then you have more ''derby'' games (Broncos, Cowboys, Titans, Warriors, Storm) which are events. Then you have a better chance (not guaranteed though) of not having a 2020 where Brisbane's only team is shit and not drawing crowds, there is at least a chance that the other team could be successful and be the ''cool'' or ''event'' team. Then there is just the sports fans that want to go to the footy each week and have a preference for RL but are attending Lions games because nothing else is on.
Finally, having two NRL teams to the AFL's one actually gives the impression that we are a RL city. The fact that SEQ has the same amount of AFL and NRL teams is a joke.