i wouldnt leave them with the jill for 12 weeks it would knacker her 7 - 8 weeks is fine imo
I agree 8 weeks is plenty long enough,i boarded a jill & 12 kits here some yrs ago & the jill came down with milk fever,she sadly died despite being trated by the vet ,the kits were as big as her & still trying to suckle, poor cow had nothing left for herself,she had done a brilliant job & paid the ultimate price