Thats odd. The rain shouldn't Rain inside the house.
Try any of the following methods:
1) Turn off Rain FX under Options and Restart the game
2) If No.1 doesn't works then backup your save file and reinstall the game
Your Save File is located in:
C:\Users\(Your Name)\AppData\Local\Shopsim\SaveGames\
There are 2 files, one is the settings and the other is the save data. Copy both of them.
3) Try making a new game... Starting a New Game most probably fixes all the bugs but loses your data. I had a couple of bugs too but when I started new game they all fixed ;D