Theory:

     Once under it, crouching in the corner, she found some shelter from the wind and the rain. Above her, the tin sheets groaned and clattered, as if they would sail away at any moment. But they were held down by the solid branch of a straggling old oak tree.

     Usha remembered that across this empty room stood an old fireplace and that there might be some shelter under the blocked-up chimney. Perhaps it would be drier than it was in her corner; but she would not attempt to find it just now. She might lose her way altogether.

     Her clothes were soaked and the water streamed down from her long black hair to form a puddle at her feet. She stamped her feet to keep them warm. She thought she heard a faint cry - was it the cat again, or an owl? – but the sound of the storm blotted out all other sounds.
 
     There had been no time to think of ghosts, but now that she was in one place, without any plans for venturing out again, she remembered Grandfather’s story about the lightning - blasted ruins. She hoped and prayed that lightning would not strike her as she sheltered there.

     Thunder boomed over the hills, and the lightning came quicker now, only a few seconds between each burst of lightning.

     Then there was a bigger flash than most, and for a second or two the entire ruin was lit up. A streak of blue sizzled along the floor of the building, in at one end and out at the other. Usha was staring straight ahead. As the opposite wall was lit up, she saw, crouching in the disused fireplace, two small figures – they could only have been children!

     The ghostly figures looked up, staring back at Usha. And then everything was dark again.
Explanation:
 
Usha tried to find a place away from wind and rain under the roof. The tin sheets made loads of noise, and it appeared as if it would blow away, but the tin sheet was held down by a branch of an oak tree.

Usha remembered that there is a chimney over there since it was clogged, she can take shelter under it. It would be drier over there, and she wanted to find it. But, she just wanted to take some time; otherwise, she might lose the way altogether.

Usha was wet, her clothes were soaked, and water tripped from her hair. She stamped her feet to keep herself warm. She again heard a faint cry, maybe again the same cat or an owl, but the sound of the storm drowned out other noises.
 
Till this time, Usha did not remember about any ghosts, as she was doing things unplanned. But since she is in one place, she suddenly remembered Grandfather's story about the ruins. She just prayed that lightning should not strike there till she takes shelter.

There was heavy thunder, and lightning struck almost every few seconds now.

Then there was a bigger flash, and the entire ruin was lit-up for a second or two. A blue streak blazed along the floor from one end to another. Usha was looking straight ahead. On one wall, she could see two small figures in the unused fireplace. She was sure they were the brother and sister.
 
The ghosts stared at Usha, and everything was dark again.
 
Meanings of difficult words:
 
Numbers
Word
Meanings
1.
Crouching
A position where the knees are bent and the upper body is brought forward and down.
2.
Straggling Be laid out in an irregular, untidy way..
3.
Blotted Stained.
4.
Venturing
Progressing.
5.
Sizzled  Blazing.
6.
Strike Hit.
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-7. The Wind on Haunted Hill by Ruskin Bond  (pp. 25-34). Published by the Tamil Nadu Textbook and Educational Services Corporation.