I have a question I have an ex-girlfriend that I used to date about 3 1/2 years ago we broke up she got married a couple of months later she got pregnant and had twins I saw her on day and she said she had twins by here husband now 3 1/2 years later she is trying to get child support from me saying they are my kids is this legal they have his last name

Its legal for her to try. If a paternity tests proves you are the father, then its legal for her to collect.

Doesn't matter what last name they have unless there was a formal adoption.