I don't know what to do

Yesterday afternoon a half starved Siamese kitten showed up at my door. I think she's a girl - no boy bits and she's a torti. I fed her some wet yesterday because she was starving. She stayed outside our apartment last night (and it rained really hard - we share a covered porch with our neighbors). My DH was not happy because I fed her so she's still here. She does have a collar on but it looks a little tight and ragged so I'm thinking she may have been dumped. I live in an apartment complex and people dump their cats all the time.
So today it looked like she was staying so I gave her some dry food and more water (I feel bad all I have is weight management). I saw my neighbor yesterday and she saw that I had fed it. Well Dh and I ran into her today and apparently the cat was scratching on their door (she did it to us too) and mat all night so her husband threatened to take her to the pound. Fortunately his wife said no that she thought we were taking care of it. So I told my husband that if she is still here tonight I will put out a box with some old blankets in it so she can stay warm (we live in the south so it's not very cold) We told her we will take care of her so don't do anything.
I called the no kill shelter and they don't take lost pets (the county animal shelter is a kill facility so there is NO WAY she is going there). I called the closest vet and told them I found a kitten. I also went to the office and told them if someone asked about a kitten that she is at my apartment.
So now I'm at a loss. I can't keep her because she is a female and mine is a female who will not accept another female. Also we are probably going to be adopting a male from my vet. I really can't bring her in because I have no idea what she may have and I can't risk exposing my cat to anything. Dh and I talked it over a little bit today and what we may do is contact a local private rescue group and offer to foster her. We will pay to get her fixed, tested and checked out. I should add that this is the worst possible week for this; both DH and I have a lot of stressful things going on. So now both of us are worried about this kitten. Does anyone have any ideas what I should do and/or what I can do to keep her around our place while we figure out what to do?
Sorry for the book, Iâ€[emoji]8482[/emoji]m a little upset about this. I am still hoping that she just got out and someone is looking for her.

Is there any way you could bring her inside and isolate her in one room with food, water, etc. until she can be vet checked?

I was thinking I could put her in the garage but the problem is that Abby is allowed in the gargage when I am in there so she will eventually go in there again and I don't know if this cat has anything so would it be safe? Meaning if the cat does have something would I have to worry that Abby would catch it?

I was thinking more like a second bathroom or such where the cat could be isolated and it would be pretty easy to disinfect if the cat should have something contagious.

You could contact a local Siamese rescue. The ones here will take Sia-mixes.