Maybe, but it still seems a bit thin ...
In Significant Others, Meredith said they had been divorced for 15 years., and that Alexis was 3 when her mother left. In a "Rose for Ever After" it was said that Castle had married Meredith on the rebound pretty much immediately after Kyra dumped him, which was the September after graduation. It was also stated that Kyra met him just after In a Hail of Bullets was published, and that they dated for 3 years. Assuming that he graduated in four years, that would probably make him almost 23 at the time of his first marriage, which would put his birth in 1970, rather than 1969.
Andrew Bikichky tweeted a freeze-frame of the court clerk's prop computer screen when Kate discovered she was still married. It has actual dates on it for Castle's marriages. According to that, he married Meredith in Feb 1993 and filed for divorce in Jun of 97 (final Nov 97), which actually would be 4 years. According to the same photo, he was married to Gina from July 98 to Oct 04.
So, he marries Meredith on the rebound after dating her 5 months, Alexis is born not quite a year later, and Meredith takes off when Alexis is 3, in the summer of 97. A year later Castle marries Gina.
It would make sense that he might have left grade-schooler Alexis home alone with her stepmother Gina for stretches while he went off adventuring in the name of "research" -- but that doesn't jive too well with Gina complaining that he never let her get close to Alexis.
As to Sophia, probably the year he split up with Meredith; because he supposedly used that connection for background on the CIA for the Derek Storm novels -- UNLESS his affair with her contributed to the breakup of his marriage to Gina, which gives us the twist of making it clear that he does not have a clean record of marital fidelity. Which is my guess about what one of his dirty secrets is ... the show has implied by omission all along that his wives cheated on him and not the other way around, but what if that isn't true? Will Kate doubt his integrity more than before if she KNOWS that he cheated during a previous marriage?
Ah, well, time will tell, and these details probably matter not one bit. As my DH likes to say, "THAT detail is what strains your suspension of disbelief?