Sorry to hear about your Dad and Grandad, what a difficult time this must be for you OP.
Easier said than done but try not to stress too much about this and focus on your family and your health. If your employer is a large one this is likely to be standard practice and the letter may be a template one drawn up by HR. In my workplace we try to meet with people after they have been off for sick for one month or more, so although a week in could be seen as a little premature, if you are likely to be off for longer they might simply be trying to be organised?
They are likely (as mummyretired says) to want to understand how long you are likely to be off (so they can plan how to cover your work), whether your stress is in any way work-related and if so whether there's anything they can/should be doing about it, whether you'll need any adjustments like change of hours or change of duties on your return. They may want to refer you to Occupational Health (again standard in many workplaces if someone is off for a longer period of time, however they define that).
If you have worked there for over 2 years, have a good performance/attendance record previously and are likely to be able to return in the near-ish future I would doubt they'll do anything more serious than perhaps give you a 'warning' that if your absence continues long-term they may need to take further steps (I don't like using the phrase 'warning' for long-term sick as it implies there's some kind of conduct issue, but legally they may feel obliged to warn you). I would ask to see a copy of your workplace's sickness absence policy as that should explain the procedure if someone is off long-term sick.
I would definitely take a union rep or a colleague along with you to the meeting, just for emotional support as much as anything else. You could also ask if they'll consider doing the meeting over the phone if that's easier for you? I'm usually happy to do that for my staff....