Yes you need to be sensible about this! Taking a test is not difficult - and they will test you for free at the Dr or family health clinic. There could be all sorts of reasons for your period not to have started. You CANNOT rely on symptoms - you need to know for sure.
Your other concerns about your ex boyfriend, his ex girlfriend and all her children are utterly irrelevant. What matters is this potential baby you think you may be carrying.
You must arrange a test. If you have not had a period for 2 months, you may already be 10 weeks pregnant. If you are considering a termintation, you only have about one to two more weeks to do this in. Even if you are not considering a termination, you NEED the facts.
Please let us know how you get on, and please DO tell a parent or guardian. You are very young, and will need support.