My brother has twins. now in their late twenties.
Both he and his wife always said that they didn't really want children at all, so to find out first of all that she was pregnant and after the first scan , that it was twins, was a big shock to put it mildly!
My sister in law had absolutely no experience with babies. The hospital they were born in, were worse than useless as each time a member of staff came in, they told her something completely different- until I spoke to the sister on the post natal ward and requested that she was specialled, so only had to deal with one nurse each shift.
Luckily, her dad had a job that meant he could pop in throughout the day to help with the feeds(she couldn't breastfeed) and on days off my sister used to go and take her out SiL and brother couldn't drive - he had a motorbike.
Both sets of parents helped out, we had them come to stay for weekends from when they were quite tiny, if only to give the parents time to catch up on sleep!
Best advice I can offer is to help out as much as poss, the parents should take whatever help is offered , especially in the first 12 months.
I found that "twice is nice" definitely true even with the extra work involved. It has been an absolute joy watching them grow up into wonderful adults with their totally different personalities!