Hi everyone! I'm new here and just after a bit of advice and to gain some understanding really. So a bit of background. I have a stepdaughter 11 and a bio daughter 4. I'm with Stepdaughters Dad who is also bio daughters Dad and stepdaughter stays with us every other weekend. This arrangements been going on about 18 months now. We used live much further away so Dad would visit stepdaughter and she only came to stay very occasionally. I only actually met her when my daughter was born and she was 7 so I actually feel like I'm still getting used to this arrangement.
Anyway my issue is that stepdaughter is very possessive of her sister to the point that Ivfeel like she doesn't like me having any alone time with her at all. If I'm upstairs and my daughter comes up to find me my stepdaughter will immediately make something up to bring my daughter back to her like 'oh my gosh (name) come and see this' and it will be something so silly and made up. If we're out and my daughter wants to go to the toilet stepdaughter will firstly try and take her on her own or if my daughter wants me take stepdaughter will creep in a minute after us even if she's only just been to the toilet herself. My instincts say it's insecurity but I'm finding it really hard to know how to deal with it.