When universities are applying contextual flags that actually mean something, for example for contextual offers or financial support, they usually require evidence of the student's caring duties, and the impact undertaking these duties has had on them, from an authorative and independent source such as social services or a GP. They can withhold the flag, and a decision on the student's application, until they receive this evidence.
Keep in mind that the school has a limited number of words for their reference and should aim to demonstrate the student's academic ability and suitability for the courses they are applying for. It should focus, without lying or significant omission, on the positives.
Contextual information should be provided where it gives an explanation for the student's academic performance or demonstrates that they have overcome significant adversity in their achievements. This should be presented factually, with clear evidence. It should not be presented as an excuse, but as an explanation. Excuses come across as negative, they draw focus to what the student has not done, not the great things that they have done.
You, your DD, and the school will need to consider whether the things that might be mentioned in relation to her siblings are actually the most significant, relevant, interesting or positive things that can be said about her. Do they tell a university that if she is given an opportunity she will make the very best of it, that she will be a great student? If not, use the words to say something better.