I don't think that Frankie being a new addition would be the cause of her going into heat late, but if she was really stressed out it may play a small role.

When was her last heat? Not every cycle for a dog is exact, she can be in heat, go in heat 5 months later, the next time it may be 7 months, then 6 months etc... It is generally about 5.5-7 months apart, but can vary slightly.

Is there any early signs that she has displayed before? Often times a few weeks before the cycle starts some dogs will show certain signs, like mounting objects, becoming more aggressive etc... If so is she showing any of those signs now?

Your vet sure seems knowledgable! I'd wait it out a bit longer, if she hasn't gone into heat within 7.5 months of her last cycle then I'd get her in with the vet for an exam.