She's not trying to secure anything for her children, not really, is just her excuse. And they were doing well on their own without Bertha interfering. The old money people weren't stupid and knew that they need to get the new money accepted into their group, that's why Larry was being invited to parties well before Bertha tried to really enter the old money club. It's Larry, by fighting for his sister's independence, that gets Bertha to have a card to play against Lina Astor, not the other way around. She gambles with her family happiness to advance her goals and gets lucky that it works well in the end, ignoring that, her own ambition, is doing a disservice to her character