If you have younger siblings, don't neglect them during your engagement! If they're old enough, try to involve them in some of the activities so they feel included in all the excitement. If they're school-aged, join them for lunch one day at the cafeteria, and if you can't make it, pack their lunch with a sweet note written on their napkin as a reminder that you're thinking about them.
For college-aged sisters and brothers, send a care package full of homemade cookies and a $20 bill. College kids are masters at stretching a few extra dollars, and they'll be thrilled with the added fun money.
And if your siblings are your age or older, make like you're kids again and go play putt-putt and grab some pizza.