Signworld is a new website with BSL learning materials, including a free Lesson One. It currently has BSL Levels 1, 2 and 3; Fingerspelling; Numbers and Colours; and a Medical Specialism. You can find it, including the free lesson, here: www.signworldlearn.com
Makaton follows spoken English and you sign the key words, BSL is a language in it's own right. Some of the signs are different too. Your sister's SALT should be able to offer some help with core signs. Look at the Something Special bit of Cbeebies too, there are some signing videos there too.
Its for her DD. Shes 4 & has speech delay problems & dyslexia, so her speech therapist & nursery have started doing makaton with her & shes picked up on that very quickly so they'd like to use that as a way of communication ready for when she goes to a SN school in september.