I don't think you can convince people. Several of my friends have seen how lovely our fluff was, how much I loved it, the environmental benefits, the (potential!) cost savings, and that it wasn't hard work or disgusting. But even though they acknowledged these things, they felt it wasn't for them.
Some people think they fancy it and will 'do it after the first few weeks'. Most of these end up a lost cause
I think because there's a such a range of objections to cloth, and the bulk of parental influences are pro sposie, it is going to have a minority appeal.
I used to get so wound up by this! But in the past 4 years there has been a steady increase in availability, and my new bb has way more interested in cloth than last time, so I can only hope that continues.
Fingers crossed your bro and sil see the light and some gentle persuasion wins the day!