Hi Fresia. Welcome to the forum.

I'm not a doctor and I have no medical qualifications.

What an awful thing to happen to you and your loved ones. I can completely understand your worries and I hope I can set your mind at rest.

Brain cancers are not common, but as brain cancers go Oligodendroglioma isn't rare and it generally isn't very aggressive.  So, as you say your brother and sister had different types of tumour.  There is very little genetic link in brain cancers, apart from a few rare genetic conditions that can affect families. I'm sure the doctors tested your sister for these already and if they applied to your family you'd already know about it.  So I think it's most likely that they were victims of a cruel coincidence - a statistical fluke.

I think that in your situation you should have a non-urgent chat with your GP so he/she is aware of your situation.  Ask the GP what to look out for and whether it might be worthwhile having a scan when you're older.  I'm sure the GP will be able to reassure you and give you some good guidance.  I doubt that the GP will think it necessary to refer you for any tests if you don't show any symptoms.