Caring for a grandparent full time is a HUGE responsibility. I applaud you that you want to take on this task but there are numerous things to consider for both you and your DF. And this point is VERY important. He needs to be on board with this 100%. If he is not and is only doing this for you, the amount of care that is required will put a huge strain on your relationship.
So here is what I think from years of experience:
1) You both need to be willing to clean up accidents that you DGM may have. I'm not talking spilled coffee or food. I'm talking bowel movements or incontinence. This is huge since she has seizures. She probably loses control and will need to be cleaned. Is your DF willing to do this?
2) You need to make sure she takes her medications on a daily basis.
3) You need to make sure she goes to the doctor. You will probably need to stay on top of making those appointments, figuring out how to get her there and staying with her while there.
4) Does she have any mobility issues? Can she walk around without assistance? If not, is someone willing to get up with her in the middle of the night to take her to the bathroom? To help her during the day?
5) Is someone going to take her out during the day to run errands, get out of the house, go for a walk? Or is she going to sit and watch television all day?
6) What happens when her health deteriorates? Are you and DF going to take on that responsibility? Because let me tell you that is huge!!
OP, I'm not trying to sound harsh. My DGM moved in with us when I was 3 months old. She was fit and healthy and able to function. It was an economical decision at that time. Then when I was a teenager her health started to deteriorate. It sucks having to watch someone you love go downhill on a daily basis. I had parents that did everything they could to make her life comfortable and that included hiring sitters so we could take family vacations. We all changed diapers towards the end, helped her get out of bed, fed her meals and I even asked her if she wanted to go to the hospital or stay home when death was imminent. I was 23. I can still cry just thinking about it. I held her hand when she died. I wouldn't change a minute of it. But during that time, if my parents wanted to go out I would babysit for the night. Yep, a 22 year old babysitting their DGM.
However, when my other DGP's started to need full time care I told my parents that I couldn't handle it a second time. My parents had been relocated to CO and DGPs were in MA. Both of them had strokes. Care was just too much and it is heartbreaking. I took care of them for about 10 days until we were able to get 24 hour help. It is very expensive and it turned out that the nursing home was a better alternative for them.
OP, I truly applaud your desire to take care of your DGM. But please for your own mental health and the relationship with your DF, think long and hard about this decision. Good luck with whatever you decide.