Around a week ago, a stray cat transferred her kittens to my backyard. Since then I've been like feeding her max 4-5 biscuits (since my parents don't want her to grow too attached) and I just go stare at them for a while in the afternoons. When she's with her kittens, I've been allowed to be within a palm length, then she'd hiss (I accidentally came close while drying clothes), but she doesn't mind if I come close when she's alone. Today, I went close after sitting directly beside her for a while and stroked her head with a finger a couple of times. She kept her eyes closed, but didn't hiss or anything, nor did she try to evade (I was near the door while she had ample open space to run away). Is she too tired and is thus not reacting, or is she okay with it?
Closing her eyes is an indication the cat trusts you.
When a cat is feeling wary of its surroundings, its natural instinct is to stay alert. A cat will only relax enough to close its eyes or sleep if it feels safe enough to. Cats even communicate a certain level of friendliness by using body language that indicates its feeling relaxed and secure in its surroundings despite, or maybe because of, your presence. Closing its eyes, slowly blinking as if it's drowsy, assuming a vulnerable position such as lying in its back are some of the ways a cat expresses it is relaxed and trusts you.
It's likely she still hisses at you when with her kittens because she is still extra protective of them, which of course is perfectly natural. At the same time, she still allows you to come quite close to them before she warns you to stay away.