Is it permissible for the husband's parents and his married siblings to take money and be extravagant with it, and does a working wife have the right to demand her share to support her siblings?

If a worker takes something with the knowledge and permission of the employer, as a donation from him, there is no harm in that. As for what is taken without his knowledge, it is not permissible and must be returned to him, unless he absolves him of it. And if the wife's work in her husband's business was a donation, then she has no right to demand wages for the past. However, she has the right to demand wages for the future, and the husband should give her something even if she does not stipulate it. If there was no agreement that the work was a donation, then recourse is made to custom.