Here's a link to the 2007 Thai Constitution presently in force:
http://www.asianlii.org/th/legis/const/2007/1.html#C02 The Palace Law on Succession was put into place in the early 20th century. It is the Kings's prerogative to change it unilaterally.
Thai Crown Prince Maha Vajiralongkorn, King Bhumibol's son, is the heir apparent. However, I do not believe that the King has designated him as his successor.
Chapter II, Section 23 of the 2007 Constitution provides in part that if the throne becomes vacant and the King has not appointed a successor,
For this purpose, the name of a Princess may be submitted.
So, yes Princess Sirindhorn could become monarch if the King fails to appoint his son as successor before death.