It depends, but for the most part, these mates are not seen positively for your Shih Tzu. Shih Tzu experts will tell you that littermates have nothing that makes you want to have it at home.
Reasons for keeping these littermates are obvious. It can be difficult for your Shih Tzu to separate from the littermates.
Do not keep the littermate even if you have more than one puppy. If you have it you should make a tough decision. It is going to take time and effort to implement it. You need to teach your puppy to live without it.
You should raise your puppy to be independent. So if you want to grow your puppy in a proper way, you should teach him to do things by himself.
So if it is possible do not get them a littermate unless you provide their independence. This is something to raise them healthy.