Age difference between Bart and Jory

No they were several years apart. Jory was 14 at the beginning of this book and Bart was 8 or 9.

Yes they were several years apart in the book, but as for this film, being a sequel and following the previous film's continuity, they were only about a year or so apart. Beginning with the film of Petals on the Wind it is established that it is 1970, yet we quickly cut to around May 1971 when Chris is graduating. Cathy is pregnant around this time and after Julian's death the film cuts to "10 Months Later", which would put the remainder of the film in 1972. Jory has just been born at this time. Cathy is pregnant with Bart at the Foxworth Christmas party that year, which would put Bart being born in 1973. So, no, the age-difference does not fit in If There Be Thorns. Also in ITBT, they establish that Bart is 12, which would put the film in 1985, yet in Seeds of Yesterday the gravestone for "the woman next door" reads "1982". I could give more examples, but yes you are correct; The timeline of this film series is horrid.