Israel will only use nukes if they feel that they cannot win they will loose Israel.
Which you suspect that will
It's highly unlikely that ISIS would use the "Storm through the gate" approach here. The point of this group is to first destabilize peace, may be by detonating some human bombs within the country.
First. Do not underestimate the efficiency of Israeli intelligence. There is a high chance that they escalated the shit so Iraq and Syria would rot within
Or when the time is right IAF will come into action. If it means killing 100 civilians to kill one ISIS
Second, if the somehow slip through and blast a bomb or two, Its nothing like that havent happened before right ?
Anyway we call just guess. Lets see the real deal in future if ISIS make that far
But in the meantime. I wouldnt underestimate Israel for they have pulled some magic rabbit out of their hats before