Lord Zeromus do you have big or small hands?
The smaller, the easyer to achieve succes.
If her pubic bones are small, this should make things really difficult or even not possible.
About convincing her, that is what i think:
That is not much about convincing her, first of all you should know how to make her cum a lot with your fingers, then you should gently insert more fingers gradualy, start with two then add more fingers as the resintance decreases. Use your two hands, one finger from each, and them more fingers from each hand and strech her gently.
All this is about her pleasure, she needs to have a good experience, never forget the clit.
After you can insert four fingers easily you should force your knuckles, no thumb yet, after the knuckles you can work the five fingers togheter.
Streching should be pleasurable to her, it's easyer to convince her about fisting after some good and messy sessions .
If you can convince her, inflatable toys could help but they should be used with care cause they can injury her. Gradualy larger dildos can help a lot too.
Dont insist about fisting her yet, play with your fingers, make her cum many times that way, and maybe the fingering can become fisting.
This is a process that take months to achieve, and even If you have really big hands and she have a narrow pelvic bone that makes impossible for fisting, you should strech her good and gently just because it's pleasurable If it's done the right way. It's all about making her having a good time.
Good luck.
P.s. :When she have room for it insert thingers while fucking her to, and alternate fucking and fingering. Dont forget to play with your thingers around her cervix too.