Would the person who recommended a series or film to her sisters, describing it as good, be sinful if they then watched it, even though she did not ask them to watch it?

You must repent to Allah Almighty for enticing others to watch movies if they included what is impermissible شرعًا (religiously), and advise yourself and your sisters to stay away from them. If you repent, what resulted from your words will not harm you, for "the one who repents from a sin is like one who has no sin," and whoever repents after causing corruption has fulfilled their duty, even if the effect of their corruption remains.