Q. "My younger sister is expecting Baby No. Two any day now, and her first child is only 16 months old.

"She has been unable to find more than a few constructive ideas in the books on child care. One even ends the parapraph with this statement: Quite simply, it is better not to get into such a situatin at all'.

"But she is in this situation. Any advice or references for pertinent reading material would be greatly appreciated."

A. You're writng to the voice of experience. Having had two children just that close (and planned, at that), we find it fairly easy to reassure your sister.

Yes, her job will be more chaotic at times, and certainly more exhausting and more challenging -- but the best jobs usually are.

There are so many pluses.

Your sister will be much more efficient than she is now, for she is having her second child soon enough to remember what techniques and shortcuts worked best with the first. It's amazing how much is forgotten in only 2-3 years.

She also is cutting down on the number of years that diapers are scattered about the house. That is a comfort, too.

And then there's the housework. It takes so much energy to nurture two young children that your sister will learn to skip most of the dreary chores and share a lot of the rest with her husband. With luck, such a sensible attitude wll last for the rest of her life.

She also will share the parental duties with her husband, much more important than sharing the chores.

Unless she works, the woman who has her children widely spaced (or who has only one child) often tries to give each child almost all the attention that's needed. This is a mistake. A baby will survive with only one parent, but he does a lot better with two.

The quality of motherhood will be richer because the parents see how much the children are alike, and yet how different. The more obvious the contrast, the more they will treat them as individuals, and this reinforces their self-esteem.

And there is also the friendship -- a joy to watch -- that blooms between these small people. Most brothers and sisters must wait years, rathr than months, for their interests to coincide.

There are minuses, of course.

Running a household with two little babies is at least as demanding as running a business, especially when the workers are rioting. Sometimes your sister will reach a point when life is not only a bowl of oatmeal, but cherries always seem out of season. That's nothing new with chldren. Your sister won't be facing more problems; she'll just be facing them in a more compressed way.

Jealousy is at the heart of most of them. The older child will feel dumped. That hurts -- and always will -- while the baby will accept the big sister or brother most of the time as a fact of life, and yet be quite jealous of the next child born.

Snapshots will help the firstborn; your sister could send some instant pictures home from the hospital. They not only show how tiny the infant is -- especially when lying alongside something familiar, like a suitcase - but the older child also gets over the initial horror of seeing his mother and father holding the baby.

To minimize the jealousy, your sister can take a specially wrapped package to the hospital: a present from the new baby to bring home. It also would be wise to have a few extra gifts at home for the older child, to present when someone brings only a present to the baby. This may seem crass, but crassness has its place in parenthood, and this is one of them. Loot won't erase jealousy, but it can keep if from getting worse.

Jealousy will continue to make the next year tough, especially in about three months when the baby starts to coo and win so much attention. By then, if not before, your sister will need an after-school sitter -- a 12-year-old is dandy -- who will take the baby out in her carriage for an hour or two while the mother spends time with the older child, reading, singing, having a tea party, taking a walk. The more individual attention each child gets, the less they will clamor for it when they are together.

That's also why the father -- or the auntie -- takes only one child at a time to the hardware store or the puppet show.

This won't leave your sister much time alone -- naps are seldom synchronized at this age -- but she can get some respite if she hires a sitter to take them out at least once a week, or to let her go out.

All difficulties are solved easier if your sister takes a few minutes to review what went wrong yesterday to figure out how to circumvent it today. The solutions will be at least a day late, but they will make her job somewhat easier. Besides, outfoxing a 2-year-old adds a little sport to the motherhood business.

And when she feels overwhelmed, suggest that she read "Mothering," by Elaine Heffner (Doubleday; $4.95), torealize again that the important jobs are always the hardest ones.