Asking family to be guest and photographer…

I would have thought that if she were happy to be the main photographer, she would already have asked if you would like her to do this. Also, it's going to be hard for her to be fully enjoy the key parts of your day (arrival of the bride, vows, speeches etc) if she is having to make sure she gets the right pictures.

I would suggest paying a photographer to at least do the ceremony/formal shots, with the hope that you will be getting a good number of informal photos from your aunt.

As you said She used to find shooting weddings stressful, then I do not think she would be comfortable shooting it for you as RomanticGreenStationery27135 says she probably would of asked you by now otherwise. But there is no harm in asking but not direct, try asking her if she can recommend anyone, asking for tips perhaps and then off the cuff "would you do it" - see what her reaction is.

On the subject of also being a guest and photographer - I have done it may times and actually would be a little - just a tincy bit if I wasn't asked. As for not being able to enjoy key parts then I would say as a photographer its a privileged position and you have a "front row seat" to everything and get to spend more time with the couple than anyone else.

Quite often I am asked is it ok if my uncle tags along and takes some candid shots as he is keen - ect. I usually say yeah that's fine so long as they don't get in my way and can keep up, they start keen but cant keep up and and very shortly after I realise they have gone and then I see them at the bar with a pint