Question:
I will be in Eretz Yisrael with my entire family over Shavuos, Bezras Hashem. I am a baal teshuva while my family is not observant, I heard that if you are going with you whole family, you may only keep one day Yom Tov. Is this true or do I still keep two days?
Answer:
I am not famiiliar with this idea. According to the Ashkenazi custom, even if the whole family comes to E.Yisroel for the whole Yom Tov, if they are not planning on staying and settling here, they still keep two days. There is an opinion that one keeps one day when he is in E. Yisroel, but that would be true even if you didn't come with your whole family. This opinion however is not the prevelant minhag.
Best Wishes
Sources:
Gra"z 496-11, Chacham Tzvi-20, Minchas Shlomo -19, Yom Tov Sheini Khilchoso 2-1.