Just keep ibuprofen next to the bed. They kick in quick on an empty stomach and enable you to start the day. Dose up regularly. Don't eat much, not that you want to, as that interferes with the drugs. Codeine is also great when you can get it. Tramadol less so as it does knock you for six and it's difficult to work/look after children on that.
As my mum said for years, periods are a part of life. You have to just get on with it and stop moaning about it, your sister doesn't. Try and focus on something else so the pain goes away.
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