I have two DC under 4. My sister's DD is 9, around five years older than my oldest.
Before DC's were born I took my niece out a lot, and often when DC1 was a baby she'd join us on days out too.
Obviously due to covid we haven't had any days out in a long time (when I say days out I mean fulls days to farm parks etc, not just an afternoon swimming etc). I mentioned to Dsis I was taking my kids on a day out this weekend and she said what about niece? Obviously because I had taken her out in the past she expected her to come with us this time too. Tbh I did consider bringing her with us but I decided against it for a few reasons (it's an hour round trip in the opposite direction from where we're going to get her/drop her off, it's expensive enough as it is, not wanting responsibility of another child on a day out etc)
Now I'm left feeling
A. Bad I didn't invite her
B. Like I would either need to invite my niece on any family day out in the future or keep it a secret from my sister when I'm going somewhere.
Was I BU to not invite her?