GW, As a deaf person using sign language, English is not her mother tongue/native language. Sign language is a real language entirely separate from spoken languages with its own grammar, sintax and so on. It is not a visual form of English. Presumably the environmental factor is that her household speaks in ASL.
To her, reading and writing in English is the same as a hearing American reading and writing in, say, French or Italian. Add the extra difficulty that written language relies heavily on matching sounds to letters, it is very hard for deaf people to learn to read and write. To them the letters are all abstract shapes not related to sounds -almost in in the same way as Chinese has characters. So if we know that S is pronounced ssss and that the words sit, stay and stop start with s, we have a reasonable chance of guessing that the word snow will also start with s, even if we've never seen it before. However, a deaf person cannot make that connection. IQ has an effect but it is simplistic to wonder why she can't read or write well. How fluent would you become in written-only Chinese?
Of course, most do learn, and I'm sure that she will do well with support from her family.