Sounds like an amazing trip you have planned Spain and Portugal are absolutely magical, especially with two families exploring together!
Your itinerary from Madrid ➔ Seville ➔ Sintra ➔ Lisbon sounds perfect for mixing culture, history, and those dreamy beach days. Staying near Sintra is a fantastic idea it's stunning, full of fairytale castles, and close enough for easy day trips to Lisbon and the coast. I'd recommend areas like Cascais or Estoril for your beach stay — beautiful beaches, family-friendly, and just a short drive into Lisbon!
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Safe travels and enjoy every moment!