Article snapshot taken from[REDACTED] with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
Henri Wittmann (1991)
assigned DSL to the French Sign Language family because of similarities in vocabulary. Peter Atke Castberg studied deaf education in Europe for two years (1803–1805), including at l'Épée's school in Paris, and founded the first deaf school in Denmark in 1807, where Danish Sign Language (DTS) developed.
The exact relationship between DTS and Old French Sign Language (VLSF) is not known; Castberg was critical of l'Épée's 'methodical signs' and also receptive to local sign language in 1807, and may thus have introduced signs from VLSF to a pre-existing local language (or home sign(s)) rather than derived DTS from VLSF itself. In any case, Castberg introduced a one-handed manual alphabet in 1808 that was based on the Spanish manual alphabet. In 1977, the Danish Deaf Association adopted 'the international manual alphabet', which was an almost exact copy of the American manual alphabet, with minor differences and additional signs for the æ, ø and å.
Icelandic Sign Language is closer; 37% of a set of analyzed signs (Aldersson 2006) were completely different in structure and a further 16% were similar but not the same. Faroese Sign Language and Greenlandic Sign Language are more clearly dialects of DSL.
Aldersson, Russell R. and Lisa J. McEntee-Atalianis. 2007. A Lexical Comparison of Icelandic Sign Language and Danish Sign Language. Birkbeck Studies in Applied Linguistics Vol 2. A Lexical Comparison of Icelandic Sign Language and Danish Sign Language Sign Language Studies | October 1, 2008 | Aldersson, Russell R; McEntee-Atalianis, Lisa J | 700+ words
^a Sign-language names reflect the region of origin. Natural sign languages are not related to the spoken language used in the same region. For example, French Sign Language originated in France, but is not related to French. Conversely, ASL and BSL both originated in English-speaking countries but are not related to each other; ASL however is related to French Sign Language.
^b Denotes the number (if known) of languages within the family. No further information is given on these languages.