When a mum breastfeeds siblings of different ages this is known as tandem breastfeeding. She can feed her children at the same time or one after the other. Tandem breastfeeding leads to closeness and bonding, not only between mum and child but also between the siblings.
Some children wean themselves during pregnancy because the taste of the breast milk changes. Others prefer to carry on being breastfed. If there is a risk of labour starting early you should discuss the situation with your doctor.
In the last weeks of pregnancy colostrum will be produced in preparation for the new baby. The mum usually has enough milk because the law of supply and demand applies to tandem breastfeeding too. However you should always be careful to feed the baby first and make sure that each feed lasts long enough.