Hacked email or total accident

I have a Facebook account which I look at occasionaly and access via the Facebook app. However whilst reading my emails I tried to access it from n email had they had sent me. I clicked the link which opened up the Facebook log in page. The boxs were complete already so I loged in, but it was somebody else's account. It was a family member who is not my Facebook friend and has never used my phone or iPad. This person gave me an old laptop 6 months ago to practice how to use them as I'm new to computing. I used it for a short time but grew fed up as I didn't know how to use properly. Can anybody advise me on how my emails link up to her Facebook Acc and what action I should take. Cheers, Kevin