Is it obligatory to disclose to the suitor that my sister has cancer, which is still under follow-up, or is it sufficient to mention that there is a problem in her neck, namely nodular goiter?

If doctors confirm your sister's complete recovery from cancer and that it will not return, there is no need to inform the suitor. However, if the disease persists or is likely to return in a noticeable way, he must be informed of it. This is because the preponderant opinion is that it is obligatory to disclose any defect that would cause aversion in either spouse, and not informing is considered deception and fraud. The suitor must be informed of the truth: that she had cancer and underwent surgery, and the probability of its recurrence according to the doctors' statement. It is not sufficient to mention other problems that do not reveal the fundamental defect.