We use the Harvey Karp method of quieting and calming babies. He has a book called the The Happiest Baby on the Block. It really helped us with our first baby. There is a video that you can watch and learn the techniques. You can hold your baby in a football type hold and they love that. The other is Sshing, where you saying Sssh, like telling someone to be quiet. However you do this right next to their ear. It worked wonders for us. We did it and it really calmed down our baby when he would cry. The library has a video- Fort Worth library (main) and there is a book also. There may be a DVD that you could buy now, you might research it on the internet also.
We also used music and singing to our child. We have several music boxes and he loves music, so that helped calm him down too. There are some CD's for babies with ocean sounds, instrumentals, so you might try a CD player and some music that might really help. I know you said you tried it. Have you tried having the 2 and 5 year old sing to the baby. You could teach the 5 and 2 year old to sing and they could sing to the baby.
Have you tried massaging her? I've heard that can be calming. There are books on it, I've done a little bit of massaging when our baby was very young.
Do you have any Baby Einstein Cd's or DVD's, you might try those. Does she have a pacifier, does the family approve if you use a pacifier. It could be that she might need something like this.
Have you tried reading some small books to her? We've used a sound machine that really helped. It had the white noise on there and our baby loved that. Do you have a sound machine? You might want to get one and see if that will help.