Being fair, Sludge, the only real vector for your mum developing covid would be your sister visiting, that would be the only new element.

You've been keeping yourself safe, no doubt taking precautions around your mum, so whilst the law may well technically exclude you visiting afterwards, it would be no more risk of you passing covid to your mum than if your sister didn't visit - the risk would be if your sister brings it into your mums house and it then passes it on to you.

So legally, you shouldn't then visit but it reality it depends how much of a risk you're willing to take with your own health. You're hardly going to be arrested for later visiting, after al.