My mother does this. Invites herself along to everything that I and my children do. It drove my ex potty, because of how controlling she was - towards him as well as me, never mind the children.
Today, for example. She asked me if I'd mind her dog today, yesterday. I said "no" because I was planning on spending the day with my son and daughter - we were intending on going somewhere that they could run around (my son) and take photographs (my daughter... and okay, me), and where we could take both of our dogs. This morning, my mother has telephoned to announce that my father is having my daughter's puppy, but not my aging dog for the day with their puppy... and she, my daughter, my son and I are going to a National Trust house, which she's never been to (but which I took my children to the year before last). And somehow it's all been settled. She's decided, she's made her mind up... so it's happening.
"No" is not a word she understands... and if it's dared to be uttered concerning this, it'll cause an almighty row which, quite frankly, I can do without. So we'll trundle along for a quiet life, but seethe with resentment because she's muscled in on what should be a time for me to spend with my children/them to spend with their Mum.
And don't get me started on Bank Holidays, Mother's/Father's Days, Christmas, New Year, Easter, or the entirity of Summer Holidays From School.
OP... your DF might be too cowed by his parents to say "no", and the fact that his sister is amongst your crowd of friends, too... well, that's twice the manipulation being applied, I'm afraid. Do you know what his childhood was like? Mine was horrendous. My parents are both abusive - emotionally, verbally and physically - my older brothers went NC with them years ago... so I'm all that's left, my children are the only grandchildren that they see, and since they became great-grandparents not so long ago to a little one they will apparently never meet/see/know about, it's been twice as hard to put my foot down. Because I feel sorry for them. Maybe your DF and his sister feel sorry for their parents, too...?
But you need to make a stand. My ex did, and it worked until he and I separated. But I'm still grateful to him for those four years of relative peace and being able to spend time with my children (and him) without my mother dominating the entire trip out/day.