I ignore announcements for local kids if we don't have a strong connection, and if there's no invitation to the party. You say "thank God" you weren't invited - is that because there would be more pressure to solve the gift problem? I thought it might be because you don't like them all that much, but you say you like and are friendly with the mom.
If you think they are trolling for gifts and you give in, that just encourages them. I resent being shaken down for gifts. I'm sort of baffled about you not being invited to the party unless by "friends" you mean only friends of the kids and no neighbors or other adults outside the family.
Your situation is more complicated because of the business referrals and the fact that one kid was your sitter.
I don't think the "graduating with honors" makes any difference - but your relationship with the older girl DOES make a difference. I think giving $50 to both is too much - if my son were in that situation and someone gave him $50 when he didn't know or speak to them, I'd be uncomfortable.
I'd give more to the one who was your sitter, frankly - you can make it from your children if that helps. If you feel the younger one needs something, why not give $20.13 since she graduated in 2013? It keeps the gift closer to $20 but makes it sort of "significant" due to the digits - it's a way to sort of justify the amount. Since the older one is going out on her own, you could give a gift plus cash - something for her house? Another option for the younger one, if she's going to college, is a gift card for her college bookstore or some sort of small gift from there - ID card holder, key chain, t-shirt, coffee mug, etc.