DS is 2.5 and still has only a few, indistinct words. We've done as follows:
Hearing test. The HV referred us. It was totally non-invasive and not at all unpleasant, and ruled out any problems - so if your DSis is worried it would be an ordeal, it's not.
Speech therapy drop in sessions - we've been to a couple. We also get good support from the SENCO sat his preschool. And they all recommend signing, which he has taken to pretty well. Speech wise he can say 'yes', 'tractor', 'down', 'go', 'drink', 'doggie' and 'Dada', but he can sign Mummy, Daddy, car, bike, friends, his name, love, home, dog and eat - he's picked all those up in a few weeks. It's relieved his frustration no end. We learn from Mr Tumble so do encourage that.
We concentrate a lot on eye contact - DS has also recently got glasses and I'm sure he's making more eye contact now. Eye contact is essential (I'm told) as one of the foundations of speech so games to encourage that are great. Also games that have turn taking (I'll put a piece of the puzzle in, you put a piece in) and ones where you can do 'ready steady go', like building towers, then knocking them down. You do the 'ready, steady...' and see if they'll do the 'GO!'.
These are all things your DSis can do that are fun and easy, and will keep him occupied, but will also potentially help his speech. He's still only two, it may not be a problem anyway, but these sorts of things will do no harm and might do some good. And seeing your child sign 'I love you' for the first time is pretty bloody adorable, so worth it just for that IMO 