from video I saw today of Galveston - everything west of 11 mile road is under water... the farther west you go on the island the deeper the water. The most populated area of the island (the east end) suffered flooding during the storm but looks like it has receded now.
If you niece’s home is on the east end of the island (not the far east end) then her home should be ok.
Jeffers posted a picture of the island on this thread that shows what areas on the island are reachable and what areas are not.