Well, you can't "fake" a connection with children, and people shouldn't expect you to! Connections come from personal events like birth. If you have had no "personal" connection with these kids, then how would that happen? But, that does not apply to how they should be treated compared to the others. Kids notice everything! They compare everything! If someone got 12 presents, and they got 11, they "will" notice it and they will draw their own conclusions as a result.
We adopted five kids! The first one was from inside her family. The other 4 were our foster kids.
There are family members that make a point of remembering "every" birthday and holiday equally, and then there are those that don't. This is overlooked by most on birthdays but Christmas is a tough one! We have made it clear to all that this will not happen again because we are the ones that are left to explain it!
You try telling 4 kids after Christmas that "they" didn't get a "special" present because "they" are not a "biological" member of the family when they have no contact with "their" biological family! It's heart breaking to watch even if they don't show disappointment!
Look! I'm not trying to dump this in your lap like your son did, but these kids are going to be a part of your life long after you are dead and gone. So, how do you want to be remembered? Will it be Grandma and Grandpa? Or will it be Daddy's parents! It's your choice!
My advice, spend some personal time with the step kids one-on-one and get to know them, you just might find someone you really do care about!