Puppy training issues - help greatly needed and appreciated

Hello, sorry if this isn’t worded well, but I would REALLY appreciate any help 🙏 - lest the little pupper be sent back;;;

My house of 3 humans has semi-recently taken in a new dog from a family member (who is a breeder), turning the number of dogs in our house to a total of 4 (each being 11, 5, 1, and 1 year(s) old respectively, if I recall correctly - all chihuahuas )

The the newest one is over a year old and is from the same litter as the pupper we welcomed before her, and she (newest addition) is not training well… she understands when we tell her to go outside, but she doesn’t understand that she’s supposed to pee and such outside - she just pees whenever and wherever. It’s also difficult bc she doesn’t flag (like, run back and forth being peeing - any behaviour she usually doesn’t do that signals she has to pee) and will just pee, meaning we can’t really be preventative or sometimes not notice until after when we can’t really tell her off anymore because she won’t understand

If we catch her at the time, she will sometimes look bad afterwards, but still hasn’t learned - the others are all fine and ask to go outside

What I’m really asking is if there is any tips on how to train and over a year old puppy to not pee inside and to ask to go outside? Especially after being used to that for over a year. My mum (I live with my parents) is seriously thinking on sending her back, but it’s just not a nice life at all (a lot of time in box crates, each taking turns being outside - since hehe has likely over 30, also all chihuahuas), and I just couldn’t bare sending her back to that, and she has also grown attached to me and me to her

Again, sorry if this is poorly worded - I will happily answer any questions! I just really need the help 🙏 thank you