Hi - retired TA (male) here, with 20 yrs KS1 classroom experience -
OH DEAR! - this all gets so sad, that so many parents worry and fret about things when there is TOTALLY NO NEED TO!
As all the other replies have said, lovessummer , your twins are perfectly within where one would be happy for them to be at the end of Reception. I have worked with plenty of children who are at this stage in Yr2, because their parents NEVER read TO or WITH them, and see school as being solely responsible for teaching, and it's not parents' job. BUT, needless to say, they are NOT MN parents!
Rather surprisingly, we haven't had the dreaded 'phonics' debate yet in this thread : do try to ensure they are sounding out correctly, and not just remembering words which have been previously told to them. I once did voluntary help with a Reception class, where many of the children would just 'recite' whole books, without even glancing at the pages, because they had done them so many times and knew them off by heart! That is NOT reading.
So, OP, carry on just as you are : a variety of books, plenty of praise, encouragement, and discussion, and above all, MUTUAL ENJOYMENT!
[ Blueskiesandbuttercups : you upset me! : S&A has always been my favourite book; it contains history, natural history, teaches you to sail, and is good clean fun, though admittedly old fashioned. 'Coot Club', set in The Broads, and can be read in conjunction with the 2-1/2" OS map, as all the locations are real. ]