Oh, this is a tricky one! Lots of parents face this issue, and I can see the dilemma. While you want both of your children to play together and grow closer, you also want to keep them both safe. The best advice I can give is just to be there while your children play.
Keep a watchful eye on both of them, and if you’re older daughter is old enough, have her also look out for her younger sister. Don’t put all the responsibility on her, but try to instill a sense of duty and protectiveness towards her little sister. And if the little sister is old enough, teach her how to properly play with small toys. Also, teach both your daughters how to deal with broken or damaged toys. If a toy breaks into many pieces, they should immediately tell an adult and be careful of the area as to not injure themselves.