"I think your problem is that your employer does not have to offer you flexible working. You have the right to request it, they do not have to give it to you."
They have already given it to her. The working pattern she requested is now her permanent terms and conditions.
Indirect sex discrimination happens when a general policy or practice is applied to everyone but disproportionately affects women. A blanket ban on flexible working or something would be indirect discrimination because although clearly it's not only women who have children
women are more likely to have caring responsibilities for children.
You can't be treated less favourably for requesting flexible working, such as being disciplined for wanting to stick to the hours agreed and refusing to agree a proposed change to your terms and conditions. The problem is you still don't have two years' service, which means they could technically just give you notice to terminate your employment and then reengage you on the terms they want. You'd then be relying on claiming discrimination as a remedy.
I would suggest writing back to your line manager, acknowledging their request to vary your terms and conditions of employment, and saying that you are not able to consent to their proposal to change your terms and conditions at this time as you have caring responsibilities including for a disabled family member, and therefore will be retaining your current terms and conditions.
Remain very polite and helpful where at all possible, but be clear that you are not giving consent to their proposed change to your terms and conditions. If you force them to either terminate your employment or actually discipline you to try and get what they want, they are more likely to back down. Obviously say things like you are concerned this could be construed as less favourable treatment because of your responsibilities for a disabled family member and for requesting flexible working, neither of which are lawful.