I have namechanged for this as it is potentially outing.
I own a property with a small mortgage which my dsis lives in, she has lived there for approximately five years. Dsis pays just enough rent to cover the mortgage and costs such as buildings insurance, which is around half of the amount that it would fetch on the open rental market. This situation came about as we were due to move out when dsis needed somewhere to live, so we held of renting or selling it and she moved in. This is a family sized home and dsis lives there by herself.
The aibu is that i have asked dsis if i would be able to use the large garden on the house for my baby dc's birthday party as it has large patio doors leading from the kitchen and a decking area, our own garden is currently in the process of being done up. It would be a small family party with no small children from outside of the family, so no chance of her possessions being ruined. I asked her as a favour as i am doing her a massive favour by letting her live there, her response was that no you can't as i dont want people walking on my grass, technically its not even her grass!
Is she being unreasonable or should i be treating her like any other tenant, even though she is my sister? Am i expecting too much from her and being cheeky? I need an outsiders perspective on this please! I am prepared to accept that i may be being unreasonable.