Recently I had no problem recieving my second 90 day pass on a first world passport, with less than a month inbetweeen the two trips to SG.
Interesting thread & comments here:
"I have a friend in the immigration in Singapore and he often said we tolerate freelance Thai prostitutes because that's what foreign visitors to Singapore like/need. They want fun and Singapore makes it possible to them, thanks to Tourist dollars.
"They don't get a yellow card which is the official registration documents for prostitutes but they can freelance around in Singapore....
"If your SIL often flies to Singapore, specially during working days she might not have a regular job. The Singapore government encourages tourism with tourists that have the money to spend. If someone flies in all the time during working weeks it could ring an alarm bell. They love tourists from Thailand that are born in Bangkok, Phuket, etc but don't really like Thai tourists born in Isaan or the North."
http://www.thaivisa.com/forum/topic/677 ... ore/page-2