When to put a baby in with her sister?

[Bobbin12]

DD2 is 8 months old and still not sleeping through. I think she needs to get into a bigger cot but there is no room in our bedroom and I am worried about putting her in with DD1 (nearly 3) incase I then have 2 DDs not sleeping instead of just 1! Does anyone have any ideas?

IME the older child quickly learns to just sleep through the smaller one waking,that has been true for a lot of my friends and for me.We all still co-sleep but dd1 (5) very rarely wakes unless dd2 (2y 10m)is talking really loudly and even then she goes back to sleep easily.

Thanks for that. How old was your dd1 when you put them in together?