Js11, Be careful, you are starting to negotiate, by offering concession after concession. Your son knows he has got you over a barrel and he is holding out until you entirely capitulate.
Go straight back to your starting position, if DS wants baby sitting, it must be arranged well in advance and it should include car transport there and back, unless you are staying overnight.
It is clear that you are one of those loving mothers who have over indulged their adult children, from the best of motives. Now you need to roll back.
There is another thread running on sibling rivalry. How do you think your younger son will react as he grow older, knowing that he always came in second behind his older brother. You may not intend to treat your older son as your favourite, but that is how it will look to the younger one.
And as we have seen in the RF, recently, hell has no fury like a younger son who thinks he has been spurned.