Me n my sister have fallen out ( nearly 4 years now ) in this time she has had twins and now another son , I'm not allowed 2 c any of them what if anything can I do

Nothing, law-wise,anyway.

Try to make up with your sister if you want to know your nephews (and niece, if one of the twins is a girl).

Aunts and Uncles , at least in the US, have no legal visitation rights to their niece or nephews.

So if you want to see them, you need to work out the issues.