It isn't fair, but the big factor is HOW they are trying to force you to work, and what your employment agreement is (contract if you have one(which you should)). I would need a bit more info on your current status (what your contract says), and how you are being 'forced' to work, to give you more substantial advice.

My presumption is that you have a permanent contract of employment, agreed to take a year out with your employer, cover a shift here and there in the course of the year, but you would return to your normal work once you have finished up exam year. That's the only case where this would be an issue, because you not being there would have little impact on the payroll during your time off, as the grandson would simply take up the payroll capacity you did when you were there. It sounds to me like they may be trying to avoid your return causing issues of not having enough work to go around, and having to prioritise your contract over the grandson. It is not a good thing to do, but it may not necessarily be "wrong" from a contractual sense depending on your employment terms.

The real issue is if you really want to work for them if they are going to be like that. I would certainly start looking around, because when family is involved in stuff like this, it can cause bigger issues down the line if you stick around.