DD1 was 3 when DD2 was born. She would sit in bed with pillows and hold her while supervised. She was allowed to stand on a stool and help me wash DD2 in the baby bath and help give her massages with baby massage oil. She would also also lie on the floor with her and play gently. All of this was supervised and I would show her how to do it.

It helped that DD2 would only sleep in the sling on me during the day for the first 8 weeks so she was on me for the most part. I think it's good to involve them but ensure they are supervised and understand what they are and are not allowed to do with the baby.