All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Indeed, you did well by not celebrating your birthday and not participating in the birthday of your colleagues or other polytheistic [non-Muslim] festivals. We ask Allaah to reward you for doing so.
For more benefit on the prohibition of exchanging gifts with non-Muslims on their religious festivals, please refer to Fatwa 89254.
However, it is permissible for you to benefit from the chocolates which were gifted to you.
Since you are prohibited from attending their festivals, you are also prohibited from attending any ritual related to such celebrations, like cutting the cake and the like.
The fact that they are non-Muslims does not allow you to attend their celebrations.
Disregarding some of the religious principles by a Muslim is not a way of influencing non-Muslims. On the contrary, if a Muslim holds fast to the principles of his religion, he may influence them better.
For more benefit, please refer to Fataawa 81725 and 86137.
As regards taking pictures of the female members of the family with a mobile phone and sending them to your brother, then there are some prohibitions on this. It might be that the wife of his brother is included in these pictures, and if we presume that every single one of them is a female Mahram of his, then these pictures might fall in the hands of a person who is not permitted to look at them.
Therefore, it is not permissible for you to take their pictures.
Allaah Knows best.