Of course parents and siblings are welcome to be in the photos, although it’s not obligatory, as sometimes new mums and dads don’t feel particularly camera ready. It’s totally up to you.
Where siblings are still young themselves, I recommend they join us at the end of the session so they can be in photographs without getting bored waiting around. If you’re bringing them along for the whole of the shoot, definitely bring them something to keep them entertained. Just as a note, we don’t have very good mobile phone signal here, so it’s worth downloading anything you want before you come if you’re planning to use phones or tablets to play movies or games.