Difficult, sometimes it works, sometimes it doesn't.
In my personal experience getting another cat has never been something I have regretted. In the case of my boycat Cashie, getting him a little girl companion has been the best thing ever for all of us, they play, they playfight, they cuddle, they groom each other, it is a joy to watch & I know he is happier and far less dependent on me, far less clingy than he was.
Is Nina an indoor cat?
I think that because Nina has previously enjoyed the company of your old boys it is quite likely she may appreciate a companion. Even if they only tolerate each other it adds interest and variety to their lives, at best it may the making of her and the best thing you could do for her.
On the other hand if they fight or one terrifies the other there is a problem but perhaps it is worth a try?
Choose your new cat carefully though, don't get an overly confident boisterous one because that might make Nina even shyer.
Maybe a young submissive cat or an elderly one??
And of course if it all works out you'll have two happier cats, a happier you and you'll have rehomed an unwanted kitty.
Good luck, let us know what you decide.
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