My kids are 3 yrs old (april) boy and 15month girl.
We started when she was about 6months.
A couple of things that we factored, considered and did:
1. When we started our son was sound asleep. Only recently (last 6-8weeks) have we started putting her in (asleep or just about) when my son is awake but calm (usually reading).
2. At first we used VERY firm bolster pillows (we got them from the yoga studio) and put one on each side of her, giving her enough room to roll over on each side but that's it and firm pillows about 8-10 inches above and below her head.
3. once she was a confident crawler, we started just putting the bolsters on the edge of the bed.
4. We also used a dimmer light and put it at the lowest point so they could see if they had to (came in handy a couple of times) and so we could see when going in trying to find bodies...^_^
5. Everyone together has been in the family bed since day one.
6. other then books and matchbox cars (sigh) our bed is of clear of everything. Pillows and bolsters are used to create a boarder around the bed. ( don't have bed rails...you may want to consider otherwise...we are kinda hands off and low tech...I haven't seen bedrails that go the full length of the bed so it just looks like trouble to me...I don't expect you to agree)
Now when we put the youngest into bed she will crawl to him and sleep so they are touching. She will either do it right away or when she stirs.
Sometimes, we noticed, he waits until she is asleep and then will move to another part of the bed so he can sleep alone.
We do find them in some weird and tangled positions...but we have never considered it unsafe. A couple of times we hear her crying, we do run to the bedroom. it has always been because of hunger, teething, or because she can't find him.
In my opinion, one can start with the sibling bed without nail biting when your youngest is a confident crawler (and your oldest becomes bossy (I hope that makes sense))
Please feel free to ask questions and I will try to answer them. Trust your gut, know your children and how they sleep.