Final answer:
To join the sentences using a present participle, one could write 'Coming down the stairs, I saw my sister.' The present participle 'coming' is used to show simultaneous actions.
Step-by-step explanation:
The question involves joining two sentences using a present participle, which requires understanding how participle phrases work in English. A present participle is a verb form ending in -ing that can be used as an adjective or to form continuous tenses. To join the sentences 'I came down the stairs.' and 'I saw my sister', we would use 'coming down the stairs' as the participle phrase that connects them. An example of a combined sentence using present participle would be 'Coming down the stairs, I saw my sister'.
Not only does the participle describe the action of the subject, but it also allows the two events to be connected in a seamless manner. It is like saying 'As I was coming down the stairs, I saw my sister', which reflects a simultaneous occurrence of events. The present participle coming modifies the action of seeing by providing additional context.