Is what Im doing okay for this cat?

There is a neighborhood cat that has been coming to our back patio alot recently and laying in the basement windows. She is a small cat, but her stomach distends slightly and she acts nervous when I try to hold her, but she is otherwise a very friendly cat. In the past week she's been sleeping in the basement windows everyday, and I've been feeding and watering her. I suspect she is pregnant and I would really like to take her in, but we have other pets and a past experience with letting in a pregnant cat did not go well. So I built her a little shelter out of a long box lined with layers of newspaper and bed sheets, and I used another box as a roof and also lined it with newspaper. Im not 100% sure if she is pregnant or not, but I just plan on keeping an eye on her for a while. The shelter is located on our back deck which is secluded so she shouldn't be bothered. Im also wary of taking her to a cat shelter because if she *is* pregnant I dont want them to put her kittens up for adoption :( or for some reason put her down, also Im not sure wether or not she is a stray. I thought she might be someone's cat that they just let out every now and then. Anyway, I want to do what is in the best interest of her and her possible kittens. And if she *does* end up giving birth there's no doubt about it I will take her in the house, our other pets will just have to be sectioned in another part of the house for a while.