1500 GK with Sisters

This is the list im going for, ill chuck it to u guys for advise.

HQ: BC With Psycannon
6 Termies
1 Psycannon
397

Elites:6 Celestians
Multi melta
Book
Immolator With EA and Smoke
201

Troops 1: 7 GK
1 Justicar
1 incerator
235

Troops 2: 7 GK
1 justicar
225

Troops 2: 9 Sobs
1 VSS
Book
Melta gun
139

Troops 3: 9 Sobs
1 VSS
Book
Melta gun
139

Heavy support: Dreadnought
TLLC
ML
EA
Smoke
148

Say what ever u want about it, (im thinking about getting another dreadnought and loseing some points else where)

its the book of st something, pretty sure that any squads near them can get LD10

I presume it's book of st lucius, making them something similar to stubborn.

Personally I would put the sisters in rhinos and they can go tank hunting, while the grey knights follow behind laying a hail of anti-infantry fire.

The dreadnoght is cool because you need the long range anti-tank but the CC arm could be very handy.

Other than that good luck

I'd try maybe a smaller Termi squad with more psycannons as a fire base if thats the role you want them in, then one bigger PAGK squad in a LR'er (one of the most effective ways of feilding pagk)
Termies are also awesome for CC though

I think the main issue you will have is anti tank, especially heavy tanks, since you need to be close with the sisters and have no real long range anti tank other than the Dread

The book of St Lucius allows any unit with a model within 6" to use the bearer's unmodified leadership value, w/c in your case would be LD 9.

The list can be effective but you do lack mobility. I agree with both Zeal and FTE. Invest in some rhinos (w/ 5th rules, even your PAGKs can use the sisters' rhinos after the 1st turn).
And I would definitely either get a 2nd dread (they work better in pairs) or seriously consider a LR.

Good luck!