Could she be expecting?

I am new here, and have been an animal lover all my life. I am looking for some non-judgemental advice, as this is not a typical "my cat got out" situation. I am really concerned for my little Casey and have always been responsible in getting my pets spayed/neutered. 

Here's the story:
We were given 2 kittens, who were at the time about 5 months old (this was in December, and no, not for Christmas). We were told they were sisters from the same litter. We decided to use our tax return to get them fixed, but while waiting, I noticed the larger of the two was indeed NOT a female, but a male. (this was about 3 weeks ago) I never really thought to take a closer look, as the people we took them from are friends of ours and I trusted their judgement. We have been keeping them separate since we found out. To our dismay, that same weekend I was looking over the female and noticed enlarged nipples. Since then, her belly has gotten droopy, and she is showing some signs of being in heat. She isn't yowling, trilling or trying to get out.. (they're not outdoor cats anyway) but there is some clear discharge bubbling out of her vagina here and there. She threw up last week, and has def been lounging around more than normal the past couple weeks. I read that in some cases of pregnancy, this could be a first sign of labor? Could she be pregnant or could this be puberty? I am so confused. She is only about 8 months old, is it even possible for her to be pregnant so young? 

I feel so stupid for not realizing they weren't both female..

(p.s our male cat will be getting fixed next week, as we finally got some of our return back) Thank you for any advice!!! 

Yes, sadly, female kittens can get pregnant as young as 4 months, although males usually aren't fertile that young. But most males are fertile by 6 months, so 2 months ago. . .yes, she's probably almost due . I hope the kittens don't have any problems from being inbred (usually it takes a few generations to see big problems, so it really depends what their ancestors were up to).

Be sure to keep the kittens for about 12 weeks, as they learn a lot from their mother and siblings until that age. And try to have them spayed/neutered before adoption, or at least make a good contract with the new owners. It would be a good idea to start her on kitten food now for the extra nutrition. And remember that males can remain fertile for a while after neutering, and females can get pregnant again soon after birthing, so it would be best to keep them separated until she's spayed.

My current foster cat was only 5 months old when she got pregnant. She had 7 kittens.

Two died and she developed severe mastitis. They have been in need of supplementing as she was not producing enough milk. Another has since died and I am hand raising the surviving four.

She was actually a great momma...some young ones with first litters are not.

If she does give birth, watch to see if she's encouraging them to nurse, stimulating them and laying with them.

I've had foster momma's that wouldn't nurse unless I sat with her ..and petted her.

Thank you Levi68, for sharing about the good, bad and the ugly. It had has prepared me to hope for the best but prepare for the worst.  I've got the Emergency Vet's number, some towels, scissors and dental floss. I borrowed an x-large dog crate from a family member, whose dog used it only once or twice.. made a soft spot using an old pillow case stuffed with puppy pee pads, and lined the rest of the cage the pee pads. Got her some soft food, and put some dry kitten chow and water in a hanging bowl (the travel ones) all in her own quiet room. My husband will be stopping for some kitten formula on the way home just in case. Keeping a very close eye on her.. I home school my son, so I am hoping to be able to be with her when it happens.  I am thinking there's no more than 2 kittens, she's bigger than she was.. but not busting at the seams. She's sleeping and eating mostly, and cleaning herself a whole lot. 
 

I plan on finding homes once the kittens are born, I have two lined up for now as I don't know the exact number. I will definitely be working out a way to get them fixed. Maybe talk to vet and see if he will work out a deal? Thank you both for the help, I truly appreciate it!