Double check but he could give each sibling R100k and then donate the rest to the Mom (theres a different limit to donations to spouses) and then the Mom could also donate R100 to each.
Afaik the R100k limit applies to the donor and not the receiver. R100k to each child would trigger a 20% tax payable within three months of the date of the donation. The spouse is of course exempt from this so combined they can donate up to R200k per year, split between the kids.