Giving my sister-in-law a necklace is a common and popular gift choice. However, applicability depends on the following factors:

1. Relationship: Is your relationship with your sister-in-law close or distant? If you have a close relationship, it may be a good choice to give your sister-in-law a necklace. But if the relationship is estranged, you may need to consider other gift options.

2. Sister-in-law's personal preferences: Do you know her preferences? If she likes jewelry or often wears a necklace, it is a good choice to give her a necklace. But if you are not sure whether she likes jewelry, you can consider other gifts, such as books she likes, cosmetics or tickets for activities she is interested in.

3. Budget: What's your budget? The price of necklaces ranges from tens of yuan to several thousand yuan. Choosing the right necklace according to your budget may be an important consideration.

To sum up, if you have a close relationship with your sister-in-law, know her preferences and the budget allows, then it is a more appropriate choice to send her a necklace. But please remember that the most important thing is your sincerity and concern for her, and the gift is just an expression.