Now see after having 2ds's I can buy for boys but girls I find harder as apart from dm, dmil and gm, I've all males.
I think to be honest that's why I have gone over board as well- it's nice to buy girlie things, even though I haven't chosen them ds1 has, just the looking aspect is a novelty.
As for the pressures on their relationship etc- I'm lucky, they are both A grade students, done extremely well in their GCSE's both currently doing 5 A-Levels, and have a small group of friends which they see as often as each other, so none of the 'ditching friends' so they can see each other. Are they really serious about each other? Yes they are. Will they stay 'together forever'? I've no idea they are 16. Am I alreading 'marrying them off'? Mmmmmm that's a hard one- she's a lovely girl, bright, funny, hard working, goals set for what she wants to achieve, ensures my ds1 completes all work and doesn't slack. What more could I ask for 
I guess when I say she's 'part of the family', maybe we have different views on what that means. For me, it's means she is always welcome here, she doesn't have to tiptoe around, she can help herself to food and drink without asking, she helps out with chores when needed, she will look after ds2 if I have no one too, if I'm doing shopping she will ask for certain drinks etc. Thats how she is 'like family' to me.
This was installed in me, when dp and I got together, and that's how his parents treated me. Mine were hardly around and I had and still do have a very difficult relationship with mine (but that's a story for another day!), so I guess I'm following how I was treated, and how comfortable and welcome
I felt.
As for her family, they also have welcomed my son with open arms, and he himself commented a while ago that he feels really comfortable there and he gets treated like family- I.e- xxxxxx will you bring the washing down out of xxxxx bedroom and also Hoover the stairs lol. And as I had xxxxxx changing the bedsheets the other day, I think we are similar!
But back to presents, I asked ds1 what Christmas was like at her house, and also showed him this thread. He said her parents are really generous, and Christmas is also a big deal in their house. But I explained my reservations on gifts, and he still wanted to give them all, so we had a little pro's and con's, and he's agreed with me that I can hold back a few of the gifts, but he may give them to her spontaninsly throughout the coming months.
So basically I've looked at all of your advice, the 'go for''s and the 'OTT' comments and taken the middle road 
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Thank you everyone though for responding. I had asked dp and dm their opinions, and they basically said 'everyone knows you go OTT at Christmas' which didn't help me at all!!
As for the poster above looking for ideas for presents- Xbox points (if he has an Xbox), iTunes (again depending on device), clothes (but find out if he wears a particular 'brand' first), aftershave, shower radio, book, DVD or boxset, chocolate.........
The poster who makes 'lumps of coal'- how do you make those? I've never heard of them but sounds like an interesting gift I can install on some difficult to buy for family members! (And yes I mean blood relations).
Once again thank you all, I've appreciated ALL of your responses and have enjoyed reading them.