Per their policy, you can’t get login access to the account. However, you could try and ask for an archive of the photos.
8. HOW TO CONTACT US ABOUT A DECEASED USER
In the event of the death of an Instagram User, please contact us. We will usually conduct our communication via email; should we require any other information, we will contact you at the email address you have provided in your request.
How do I report a deceased person's account on Instagram?
It is our policy to remove the account of a deceased person from Instagram. To protect the privacy of people on Instagram, we are unable to provide anyone with login information to an account.
When you submit a report of a deceased person, we require proof of death such as a death certificate or obituary.