If I were in your shoes, I would do the following:
Assume word will get round all the family who need to know - there is no need for proper 'invitations', and the guest list is not your problem.
Assume nothing will be booked in the way of catering, music, or entertainers - if something does get booked, fine, but it is not an issue.
Assume food will be platters got from a supermarket, so a couple of days before the date offer to go and collect anything that needs collecting, and also ask how many people are coming - this conversation at the time will give you a clue if the food arrangements are inadequate with a day or two spare to help do emergency shopping if necessary.
Two or three weeks before the date, offer to make/buy a cake as your gift to them - this conversation will let you know if they already have this planned.
Assume they won't want loud music, but if they don't mention it, then a day or two before the date offer to bring a speaker for some 'quiet background music to give a nice atmosphere' - spend some time over the next few weeks putting together a long playlist for their age group or tastes, just in case it is needed.
Nearer the date, ask them what time you should arrive - this will let you know if they have planned time for setting up. Then offer to arrive early to help set up, then ask if they would like you to bring any extra balloons or other decorations, or table decorations or paper tablecloths.
Nearer the date, ask if you need to bring a jumper or will the hall be heated? - this will prompt them to think about it.
If you know the venue, see if there is any way you can visit it yourself some weeks in advance to check out the facilities. Have a good look at what the kitchen is supplied with. No need to tell them you are doing this.
Make a list of everything that could possibly be handy, like towels, bin bags, washing up liquid and sponge, dustpan and brush, sellotape and pins for decorations, extra plastic plates, cups and cutlery, paper napkins, tea or coffee supplies, cake knife, some children's toys, etc. etc. Put it all in the boot of your car on the day, don't mention it, but if it becomes apparent that something is needed just quietly go and get it from your car.
In short, you have a balancing act between letting them get on with it but offering to help with a few specific things nearer the time, and having some idea in your head of back up plans or contingencies.