You have the luxury of time.
Go out with just the cow at sunset, set the camera to AV, start at f/8. Take an image and review it. Good angle? Good shadow? move around and experiment with the exposure compensation.
What do you want in focus? Just the cows head? then a small aperture would be fine. Otherwise you might have to go to F/16. and up the ISO.
Experiment, experiment, experiment. Once you have the cow the way you want it, add the girl. 8^)
"Experiment, experiment, experiment."
You know we get this same type question a lot. The answer always is, we can't give exact settings without being there and seeing the conditions. I do a lot of Senior photos! I am in the middle of 60 Senior posters for the music department. They need to be in by a week from this Friday for Senior Night at the football game.
I would simply start with P mode. Do not use any of the automatic modes besides P mode. I would set ISO 200. Set One shot and use just the center focus point. Never use Ai-servo. Now you will have to "Experiment, experiment, experiment.". Perhaps you will have to go to Av and set the lens yourself. The ISO may need to be set higher, etc. Change you position. Shoot angles because photography is all about light and angles.
"Experiment, experiment, experiment."
BTW, here is one of the Senior posters.
They are printed on a 24" x 36" canvas. They seem to be a very popular thing anymore as most departments in the school is doing it. Most of the schools in my area do it as well. I just do the Performing Arts department. We have done them for five years in a row now. We even did last year!
I might add to your situation, I would shoot Raw and not jpg if possible. It will require you to use some form of editing software but you have access to the free form Canon DPP4. In it you can u/l the photos to your computer from the XSI. You can do as much or as little editing as you prefer. You can do nothing at all but the opportunity is there. It gives you the chance to make your once in a lifetime Senior photos the best they can be.
Let us know how it goes and come back if you need further assistance.
"ebiggs: I almost said start with P, but I figgered you would com along and say to start with AV. 8^)"
Great minds think alike! I only suggested the P mode because you did Av which is also a good place to start but the "Experiment, experiment, experiment." is the most important advice.
"As far as the editing software, I use Adobe Photoshop Elements 2021. No renting required."
As a person that does, or did (I am retired now but it sometimes doesn't seem so), photography professionally, I use and still use Photoshop. I understand the rental fee. The Canon DPP4 is very capable and will do "most" of what you might need.
There is also Photoshop Elements. It is a one time purchase (about a hundred bucks I think) and it will do, again, most of what you will ever need. It also has a catalog section where you can do all sorts of stuff moms and dads do with kid pictures. For instance its nice to have hundreds, thousands, of pictures of your kids but not if you can never find the one you want. PSE allows you to put keywords on photos and it is searchable. So, you need to find that very important "cow" shot, all you do is search for "cow". And, magically it pops up on the screen. It makes photos Facebook, plus the others ready. You can have a hard bound book made, etc. All sorts of stuff but it does require some time and learning. Nothing you can't handle.
BTW, if have some "failed" experiments*, you can post them here and we might be able to point out alternate settings.
*"failed" is in quotes, because photography like "Meet the Robinson's" can help you celebrate failure as a learning experience:
"Uncle Gaston: If I gave up every time I failed, I would have never invented the meatball cannon.
Grandpa Bud: If I gave up every time I failed, I would never have invented my fireproof pants!
[Pants burn up, revealing his underwear]
Grandpa Bud: Still working the kinks out a bit."
What editing software do you prefer? I don't want to pay monthly for one though
If you think that you might want to purchase Photoshop Elements (as kvbarkley and ebiggs1 have recommended), it would be wise to wait till the end of October to purchase. The new version - PSE 2022 - is due to be released during Adobe MAX 2021 | The Creativity Conference, Oct 26-28, 2021.