Adopter12345 I am sorry this letter has upset you. You know you are not just babysitting, you are his mum, but she is also his birth mum and she is writing as that, it must be very hard to follow rules, especially if you do not even know them.
I would be happy to speak privately through pm if it were helpful.
Our ds is 5 and has been with us for 18 months, we have an 11 year old birth dd and have received a few letters from our son's birth family.
The letters are addressed to us and we do not share then with ds, we drip feed in bits of info from the letters, e.g. your birth mum liked ice hockey when she was younger... etc. This is the style of system I think is intended, where we are, and I think this works best.
When ds is older we may move to a system where she writes direct to him and vice versa, but my gut feeling is that we may always keep this system, since it puts me (and dh) in control of what bits of letters get passed on, with the understanding when he is 18, ds will see all the letters and be free to contact his birth mum if he wishes to. Luckily, for us, our son's birth mum seems to understand the way things work, but then our county adoption services are excellent and I am aware not all areas work so well.
I would ask the letter box services to explain to her how it works and encourage her with some support in how to write.
I feel TeamAcorn has hit the nail on the head with a number of points and I agree with them.
I would also go further and say that if she is from another country her English (spoken or written or both) may not be excellent or native speaker level (and although you have met her in person and know how she coped speaking English you may not know how she copes writing in English).
So she is most probably rather traumatised at the loss of her child, whether she gave up her child freely or they were removed, regardless of how deserved the removal was, she may well sill feel very shaken by this even years later (as I know our son's birth mum is).
So to the specifics I would say that the secrecy and not wanting to say too much could be a personal thing, or a relic of being from a different country (in some countries revealing personal information is seen very differently and it may be that in her native country people are less free with information for all kinds of reasons). In order to combat this I would (in your shoes) say lots of encouraging stuff (if true) e.g. dc was pleased to hear you liked volley ball and we watched some on the telly, on line etc etc. This may make her feel willing to reveal more.
I really think in your shoes I would seek some help from someone (adoption services/adoption buddy/a good friend who has adopted) to work through these feelings induced by the letter and find some peace.
Your child does have a birth mum and therefore they have birth grandparents, whether they see these people ever again or not (and the reality is they may well never see them again). So the comment - '... all your grandparents send you kisses and love and hugs" to me says 'you have not been forgotten by your birth family.'
This could be very hard for children who were abused by birth family or birth grandparents. And if this were the case (not saying it is, of course), I would certainly feel these comments were inappropriate. But if not, then I would just read them as '... you are not forgotten and you are cared for, from afar.' I don't see it as harmful.
As others have said, I would read "Sorry my letter is short. It's just because you don't write letters to me yet. When you do, we can both write lots of long letters to each other." To mean she does not know what to say and is a bit floundering.
If it can be explained to her that the letters are from birth family to adoptive parents and vice versa she may begin to open up rather than waiting for the day when the child could write.
As I say, I do not expect my son will write letters (apart form us, who does these days!) and the danger I see in ever putting it into my ds's hands is he would say he did not want to, and I would want letter box contact to continue. So I feel for our son's birth mum to have the contact is good and it will continue as long as it is appropriate.
I am not sure if my comments will help or not. I just feel you are reading a bit more into what is there and that the birth mum is not sure how to phrase things, which could be for a whole range of reasons.
I was very unsure about contact, ours also includes photos, which I was not sure about. But it has been good and it has helped me, and I hope her too, so even though it is all for the child, actually it is helping us too. Hope this makes sense. And it is just my humble opinion.