If your brother and sister had done ‘umrah and they were travelling, there is nothing wrong with them breaking the fast, because it is permissible for the traveller to break the fast in Ramadaan, as Allaah says (interpretation of the meaning):
“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days”
But if they were not travelling and they broke their fast because of the tiredness and exhaustion that you mention and because they were afraid they might die, there is no sin on them for that, because Allaah has granted a concession allowing the sick person not to fast, so whoever finds himself experiencing such hardship comes under the same ruling as one who is sick.
In either case (whether they were travelling or not), there is no blame or sin on them, but they have to make up a day to replace that day.
As for their ‘umrah, if they did all the obligatory and essential parts of it, then it is valid in sha Allaah.
And Allaah knows best.