They have a bigger change than you think. A recent hypothetical poll put them at 10% national support (without that party actually existing or them campaigning!), and it's very likely that their spontaneous support is highly concentrated in urban areas and the red wall. That means they'r competitive in those seats. Now imagine them going in to an alliance with the Green party (=15%) and they're already bigger than the LibDems. Now if they start campaigning, and they manage to increase the turnout by 1%, steal 1% of the LibDems, and 2% of Labour and Reform each (respectively disengaged poor/working class people with whom leftist socio-economic plans resonate, dissapointed middle class Labour-voters who went to the liberals, dissapointed leftists who vote Labour because it's the best thing on the ballot, and dissapointed Labour voters who're "giving Reform a chance"), and you've suddenly got yourself the second biggest party in the election.
Try to prevent that from happening by not engaging in talks about politics with them. It's hard to convince your family, harder than convincing anyone else perhaps, try not to lose them by engaging in a futile activity.