Like said earlier. Make sure she is unstressed.
Be very demonstrative you are careful and easy when you handle her kittens too...
She do have a nest? An sideturned cardboard box can be good for a possiblility to hide a little.
Dont look directly at her, look a little at the side of her.
You may look at her halfclosing your eyes.
Standing on all four, strech yourself backwards or forwards - everything like friendly cats do meeting a pal.
ps. If she is a dumped tame homecat, it isnt nothing unusual if she is a shy in the beginning. It is entirely normal and even sound behavior.
But if she is a semiferal, her behavior of letting you pet her and handle her isnt because she is practically tame. It is a strategy to survive, herself and above everything, her kittens: she will do almost everything to help the kittens through....
So it is important not do overdo this... And be sure you show her you do understand her needs.
Although your work is of course much easier then with a agressive mom...
- Of course, really aggressive semiferals are practically never taken in. And in these rare cases, never send to inexperienced fosterers. Not by a good shelter/rescue organization.
As long she understands you are hers and her kittens best practical chance, she will cooperate and try to copy with the situation. She will even accept the kittens will be fostered as homecats. Your shouldnt have any big troubles, as long you are careful and respectful to her.