The boyfriend's daughters consistently undermine the girlfriend's interests and commitments. The pattern of scheduling trips during significant events in her life is far from coincidental; it is deliberate and malicious. The girlfriend deserves to be treated with respect and consideration, not constant exclusion. Her feelings are completely valid, and she should not be forced into situations where she is clearly unwanted and disregarded.
The girlfriend is overly sensitive and dramatic. While the boyfriend's daughters may have their flaws, the girlfriend seems to expect to be the center of attention at all times. Her insistence on always getting her way and her accusations of intentional sabotage are exaggerated. Perhaps she should consider that the daughters simply have their own lives and preferences, and it's not always about her.
The evidence suggests a pattern of disrespect and exclusion on the part of the boyfriend's daughters. The repeated scheduling conflicts and the disregard for the girlfriend's interests indicate a lack of consideration. While the girlfriend may have some minor expectations, the daughters' behavior crosses the line into deliberate sabotage, leaving the boyfriend complicit by not addressing the behaviour. Thus, the boyfriend and his daughters are in the wrong.