No problem. It usually helps to draw a grid (I have to use dots to make spaces) and put "Xs" where you have ruled things out:
David is 2 years younger than John, so John must be either 11 or 12
Gemma is 2 years younger than Jane , so Jane must be either 11 or 12
Therefore John & Jane are 11 and 12 (you don't need to know which).
Now you just need to sort out who is 9 and who is 10.
Maria is older than David, therefore David must be 9 and Maria is 10 and one of the twins.
That leaves only Gemma, and she must also be 10, so the correct answer is D - Maria and Gemma.
(I must admit that the very first time I saw that question I had to puzzle over it a bit, but I hadn't done much VR at that point!)
Hope that helps?