niece asked me to be her guardian ( coming from a different country to study in UK)

[pencilcase11]

niece asked me to be her guardian ( she is coming from a different country to study in UK). What are the complications/ responsibilities of this status for me?

How old is she?

Also depends on country of origin - you will guarantee she will only study and will leave on time. Depending on age and country depends on the consequences for you. You have to really trust the person tbh

she is from EU country and is 16

At DD's school, the overseas girls with guardians are usually boarding. The guardian collects them for exeat weekends, takes them to and from the airport at the beginning/end of terms and may go to things like parents' evenings, concerts etc if they wish. They also sign forms eg permission to go on a trip.

I would actually ask the school what the position entails. They might have a list of duties!

You are effectively in loco parentis during termtime. First port of call in an emergency or if they can't reach parents.

As an expat, we asked our DSIL to be guardian for our DCs in the boarding schools in UK. She has been great and really helped us a lot. We were in Far East when DD boarded, she went to pick DD up from school and looked after her during the short Exeat, and we flew DD back over all half terms and holidays. In general, the housemistress or housemaster will contact us first in all matters. We are the one who sign all the approval forms, and not the guardian.
Now DD is in Medical school in London, she can help out a bit with her little brother. We are now based in EU, and DS will stay with DD in London during Exeat, he can get there by train, and many occasions I just hop over and stay with them in London for the long weekend. And as usual we bring him back on all half term and holidays. We arrange taxi service from school to and from airport, so do not need to take up my DSIL time to chauffeur them around. DSIL is still his Guardian, but this time round, she does not really need to get too involve. I don't think DS school had even contact her once.
I think you need to check with your niece and her parents on what they expect of you, and how far is her school from your house, can she train to you or whether you need to pick her up to and from school.
The school will have term dates set well in advance, if it happen to crash with your schedule, you can always get her to stay with her school friends. If she is 16, most likely she will be invited to stay with friends over the Exeat.

My brother is the legal guardian for my DD. At the moment he really is used as a taxi service to and from the airport at Christmas, Easter and Summer holidays..

He will also provide a home for her on the exeat and half terms. Most of the permission letters from the school are sent via email.

Money wise my brother tends to spend as he sees fits and bills me afterwards.

Although he doesn't charge for food and board or travel expenses. He only bills for purchases..Although we have friends who are charged by their family members.