B.A.
My 20 month old (at the time) daughter was present at the birth of her younger sister. It went very well. I think the most important thing, which you already have planned for, is to have one person present whose sole responsibility is to watch the older sibling. My sister and mom were both at my birth (in addition to my midwives) and my sister was able to occupy my older daughter and see that her needs were met during my labor.
I also tend to have fast babies after a long period of slow prodromal labor so there was only a little over an hour when I was completely unable to pay attention to my older daughter. I do know that others have had older siblings present for much longer active labors though so I'm sure it would have been fine had my younger daughter taken longer.
Feel free to message me if you want any more particulars!
Best wishes!
~B.