since they can't go outside to range, you'll need extra space for their pen -- think something like 8x10, so an entire room would have to be given over to them.
I agree with both of these points.
Many people build a coop (shelter from the weather and protection from predators) and a run (larger outdoor space), but inside your apartment they are already protected from weather and predators.
So they just need one area with the correct amount of space-- probably 5 to 10 square feet per chicken. That means a space at least 5x7 feet or 6x6 feet, but bigger is MUCH better.
They can have food & water, a low roost in case they want to sleep off the floor, bedding to scratch in and deal with droppings, a dust bath.
This could meet the needs of the chickens, but it is probably not good for the people or the apartment! The noise of the chickens may or may not bother you, but the dust almost certainly will.
Also, you should expect roughly half of those Silkies to be male. That means they will start crowing sometime in the next few months, and they will try to mate with the females. If you have equal numbers of males and females, the females will get pestered and overmated, so you should plan to divide their space and put the males and females in separate pens (or find new homes for the males.) Putting just 1 male with the females is usually fine, although it depends on how many females-- 4 females will do better than 2, because the male will spread his attentions among them.
If push comes to shove, last resort might me taking them to mom's place where she has a backyard. Only thing I'm concerned with is predators and the cold NYC weather. Any good advice?
I would start working a coop in that backyard now, so you can move either some or all of the chickens there when they get bigger.
The question of how many males and how many females will matter just as much there, so you might build a coop that can be divided in half so the males can live next to the females but cannot bother them.
Since you originally wanted 2 as indoor pets, you might keep 2 or 3 as apartment pets, and move the rest to an outdoor coop at your mom's place.
Or you could wait until you can tell gender, pick a few favorites, and try to find new homes for the rest. Yes, you are attached to them, but they might also have a great life with someone else who wants a few pet chickens.
(If you keep a few, I recommend keeping females, but I know that some people do keep roosters as housepets-- just consider whether the crowing will bother the neighbors through the walls.)
Eating the extras (male or female) is also an option, although I recognize it might not be the right choice for you.