Outside cats and the winter?

Okay, so i'm gonna try to ask this here and hope I actually get "responses." I asked this on yahoo answers and was just told several times that this is abuse to my cat, it's not.

My cat is 16 1/2 years old now and has been an outside cat since around 2009. I believe she is starting to outlive an average life span for a cat, an outside  one especially. For a while she was outside and inside half of the day or so and we had a litterbox for her, well after a point the litterboxes starting making my step dad sick and said we had to put all the cats outside permanently and they've been fine out there, we have a food and water bowl and they eat several times a day. (for some the other site was under the impression I did not feed my cat at all? So figured I'd clarify, sorry.) Well it seems my winters are getting harsher here every year, I let her in the other night because I had a really bad feeling about the weather that night. It ended up below 10 that night, and that's rare where I am because i'm in the south. My step-dad often works for several hours a day or sometimes over night, so every time he goes to work I've been letting her in,  it's pretty funny because now when he lives she comes to door to be let in because she knows i've been doing that. well for the past few nights since then he comes home around 10 or 11 and that's especially when get it colds here so I can't just shove her outside, i'd feel really bad. But I can't leave her inside all day since she doesn't have a litterbox. And since i'm sneaking her in so it'd be espeically bad if she ended up using the bathroom inside. Last night I refused to put her outside and i've been sitting in my room with her all night and watching her, she kept trying to go into my closet and I couldn't figure out why. Well like it's 6am now and I haven't slept causing i've been watching her and I refuse to put her outside until it gets a little warmer. I couldn't convince my step dad to let stay in because he always tells me "Oh she's fine outside and if we let her in she's going to use the bathroom in the house.She has flees" I actually had her inside the other night for atleast 15 hours. I tried letting her outside several times and she refused, but I was pretty surprised she didn't use it anywhere.  I also really haven't noticed flees either but he's not going to budge. I just can't stay up with her all night and watch her outside because it's too cold. My step dad claims theres a place under our house that's really warm because its connected to the heater and that she goes there, but I'm still worried. It's been around 25 all night but I still think it's too cold for here.

I apologize that this is so long, I just really didn't know how to word it.

Really i'm just asking for a solution and really if she'd be okay outside below freezing tempterate, or atleast when I had to let her outside.

Sometimes I let her outside and she'll just sit by the door and meow for hours if I don't let her in and I feel really bad. 

I might've posted this in the wrong thread. I apologize. I just joined.

Old cats are very sensitive to the cold, and your cat clearly doesn't want to be outside. Geriatric cats almost always have some degree of arthritis, and the cold causes them to ache. Can you keep this cat confined to your room with a litter box? There's no reason for anyone to be bothered by the presence of a litter box if the box is kept clean. There are deodorizing brands of litter that keep it from smelling bad (provided you scoop the litter out a few times a day--you can put the used litter in brown lunch bags and then put them directly outside the house into a garbage can). If you're not able to keep this cat inside and provide it with a litter box, is there a family member who would be willing to take the cat inside their home and care for it? Younger cats can handle the cold, but older cats suffer. If you cannot keep the cat inside or find another home for her, please provide her with an outdoor shelter. There are many threads on this forum about how to make a pet house out of a Rubber maid container and fill it with straw and insulate it. If you have the money, you can purchase a pet house from Amazon or Ebay online, and some come with a heating pad. Please have a discussion with your step-father about the situation and provide him with this information--it would be better than hiding the cat from him when she's inside. Thank you for taking the time to ask for help.

I agree with MsAimee.  16 is too old to be left outside in cold weather especially when she is exhibiting behavior that she wants to stay inside.  If you are able to keep her in your room, I would do so.  Have you tried other types of litter? Maybe your stepdad is sensitive to the clay litter. I love the wheat litter, it works really well and does not produce any of the harsh fumes that clay litter does. I have a 15 year old cat that stays inside and can tell that she struggles when the temps get cold even though we keep her warm.  She loves to sit in front of the fireplace and we have a thermal blanket for her.  She is just very sensitive to the cold since she has arthritis and is older. I wish you and your kitty the best of luck. Hopefully you can figure something out.