Complex sentence: Firstly, four members of the Ruysdael family were landscapists with similar signatures, which sometimes later fraudulently have been altered into Jacob's, which typically read "JvRuisdael" or "JVR".
Split into simpler sentences: Firstly, four members of the Ruysdael family were landscapists. The works that do have a signature typically read "JvRuisdael" or "JVR".