I have been practicing in the website japanesetests4you.com and I don't know why 会わせてあげるis being used. I understand that the speaker is saying: You have a sister（あなたにはお姉さんがいるの）. She is an amazing person and a celebrity（素敵な人で有名人よ）. But what I don't understand is the last bit: 東京へ連れて行って会わせてあげる. Can somebody explain me if it is using the causative form of the verb 会う and why? 手伝ってください．文の意味が分かりません。
会わせてあげる is the causative-form of 会う, followed by the subsidiary verb あげる.
- 会う: to meet her
- 会わせる: to let you meet her
- 会わせてあげる: to let you meet her (for you)
Here, the subject (I), the causee (you) and the object (her) are all omitted because they can be inferred from the context (as well as the use of あげる), but the "full" version is:
I'll let you meet her.
The causee is marked with を because 会う is an intransitive verb. See this for the rule.
Also note that although it's called "causative", (さ)せる is translated as 'allow', 'let' or 'make' depending on the context. Without the causative-form, 会ってあげる would mean "I'll meet her (for you)", which doesn't make sense here.