What would you do?

Okay, Lemon8 fam, I need to share a 'STORYTIME' that's been weighing heavily on my mind, and I really want to know: Am I the A-hole here? My sister has this habit of asking me to babysit her kids, usually at the last minute. I adore my nieces and nephews, truly, but sometimes it feels less like a favor and more like an expectation, almost like I'm her on-call, unpaid childcare. The latest incident happened last Friday. She called me, sounding a bit frazzled, and said she absolutely needed me to watch her youngest on Saturday from morning till evening. Her reason? She had a 'super important, once-in-a-lifetime opportunity' that she couldn't miss. When I pressed for details, it was a bit vague, something about a networking event downtown that was crucial for her career. Here's my dilemma: I had already planned my entire Saturday as a dedicated self-care day. I'd been looking forward to it for weeks! I had booked myself a relaxing massage, planned to catch up on a new novel I'd bought, and even scheduled a long, uninterrupted bubble bath. These moments of peace are rare for me, and frankly, essential for my mental well-being after a stressful week. I gently told her I couldn't, explaining I had prior, non-negotiable commitments. Her reaction was immediate and, honestly, quite hurtful. She sighed dramatically, then launched into, 'But you're family! Family helps family! What could possibly be more important than helping me out, your own sister?' She even hinted that if I really cared, I'd drop everything. I felt this huge wave of guilt, coupled with a growing frustration. It's not that I don't want to help, but it feels like my time and plans are constantly seen as less important than hers. This isn't an isolated incident. It's a recurring pattern, and it's making me hesitant to even answer her calls sometimes. I love her, and I want to be a supportive sibling, but where do you draw the line between being helpful and being taken advantage of? How do you set healthy boundaries with family without causing massive rifts? I've tried explaining my need for personal time before, but it often leads to arguments or her making me feel like I'm being selfish. I'm genuinely struggling with this. On one hand, I feel the pressure of family obligation. On the other, I feel my own mental health and personal boundaries are being eroded. So, I'm putting it out there: Am I truly the A-hole for prioritizing my own self-care and plans over last-minute babysitting requests from my sister? What strategies do you use to manage family expectations and protect your own time? Any advice on how to communicate this more effectively without making things worse would be incredibly helpful. Thanks for letting me share my 'STORYTIME' and for any insights you can offer!