Here are the words or phrases that should be replaced in each sentence:

1. **think about** - should be replaced with "am considering" (for better phrasing).
2. **The siblings** - could be replaced with "They" (for smoother context).
3. **are having** - should be replaced with "have" (for correct usage).
4. **after the Christmas** - should be replaced with "for Christmas" (as we do not use "the" in this context).
5. **am not remembering the first time** - should be replaced with "do not remember the first time" (for correct verb usage).
6. **an other** - should be replaced with "another" (correct spelling).

To summarize:
1. think about → am considering
2. The siblings → They
3. are having → have
4. after the Christmas → for Christmas
5. am not remembering the first time → do not remember the first time
6. an other → another