I have had this issue always... to the extent that I thought this was normal and that you actually created them to be this useless ??? (tought the training system you have built is meant for children that start at age 0, but since siblings start at what 12 and 14? they will not get the full training)... But seeing this post, is it reallly a bug? Do people really get siblings with high stats? My game is totally unmodded so...
The oldest one, the sister, I get to train once so she always come of age with only 1 attribute point. The younger brother will get two training periods so he comes out with two attribute points... both are pretty damn useless tho compared to all other characters in the game, who have like what, 20 points?, at the age of 20...