Help! Very Tiny Kittens!!

My sister-in-law called me this morning and asked me if I wanted kittens. I thought maybe she saw some in the paper or something like that. Nope, she has 2 kittens from under her deck, and the mother is nowhere to be found. From what she says, they are smaller than her hand. I have no clue how old they are, but she thinks 2-3 weeks.
She gave them milk! I said not to, so hopefully she took it away. We are going to pick them up and take them with us to petco. There is an adoption fair going on there right now and a woman who knows a bit about this, we are also going to get the kitten formula and bottles if need be.
I couldn't just leave them with her. I will write again when we get back. I'm just hoping they are older than what she thinks, wish me luck :(
She says they have fleas too.
Also, what are the chances that these are the only 2 kittens the mother had?
Any info anyone can give me I would appreciate! Thank you.

When did she become aware they were under the porch? It's possible the mom and the rest of the litter is nearby. Mama may have been in the process of moving them to or from that location?

Ok, we just got back and there are 4 kittens, not 2. Mother is nowhere and hasn't been around for a while. SIL saw a black cat a few weeks prior to this but hasn't for some time now. They are constantly meowing so I'm sure the mother hasn't been there for a while cause they are starving. My sister-in-law has been watching for a few days and heard the kittens. She didn't want to leave them in there any longer.
They are older than what she thought. They are almost 5 weeks old, so that's good anyway. Too young to be away from their mother but without her there we just had to take them. We are giving them the formula with bottles because they will not eat the wet food yet. The woman at the rescue place said 7-10 days from now they should start eating the wet food on their own.
We gave them their baths with dish soap and found quite a few dead fleas on them, they are now resting together, all clean and full bellys.
We're going to keep them till they're on regular food and can have their shots and stuff.
I will post pics later, we took some good ones of their bath. :D

Sneaky,you are an angel...Good Luck!

Aww Sneaky you are an angel......those kitties must feel so much better being fed cleaned and warm again your a darling :)

Thank you much for the comments. :D