Op, we are close to DHs sister. We each have 2 kids, all 14-15mths apart, with her eldest being 21, my youngest almost 18.
We give £25 for birthdays and xmas, a little more at 18th (£50 I think I gave) and £100 for 21st. Nothing for holidays or exams. I think we will carry on with normal birthdays and xmas till each is settled in a job.
SIL did send mine for Easter a couple of years in a row, early teens, so we reciprocated. Not sure why she started then as we hadn't done easter beforehand (dont live close and dont usually see around Easter) and haven't done anything since.
We also exchanged Easter gifts with DSs godparents and their son for a few years, but that has dropped off as the kids have got older.
The only person I now send anything to at Easter is my cousin's 8yo DGS. My mum used to have one of my DC to stay for a week at Easter, the other in summer. She always took them to see my cousin, who would have Eggs for them at Easter, so I feel I am paying back that kindness now. I sent him a card and some money this week. I will probably stop in early teens.
My two have an envelope off PIL, which will be some cash, and that is it (well apart from what we give them and whatever the bunny brings!). My parents are no longer with us and DB and I are low contact, so haven't exchanged any presents with his family for years (his choice originally). They don't expect anything off other relatives now they are older.
I think you have been very kind up to now, and would agree with you, maybe just send youngest an Egg or some money, but stop eldest. And DO NOT give holiday money. Mine have never had that, except maybe if we have seen GPs just before a holiday. Certainly never off auntie or uncle!
In terms of buying youngest bot not others, my 20yo DS has obviously had less xmas/bday presents than DD in last couple of years, as some relatives/friends have stopped at 18. It hasn't bothered him in the slightest.