'for your loss of' is unusual. Normally, you're sorry for someone's loss, full stop.

Here I might say: I am very/so sorry to hear that you lost both your niece and your husband.

YOUR loss of - is unusual, but not THE loss of.

I am sorry for the loss of your niece and her husband.
Please accept my sincere sympathies. My warmest thoughts and my prayers go out to you at this very difficult and sad time.

Yes, I don't see why not. Be sure to write niece with IE.

It might read a little more smoothly not to repeat 'your,' to reduce the number of 'and's, and perhaps to vary the sentences a little more.

I was so (terribly) sorry to learn/hear of the (sudden) loss of your niece and her husband. Please accept my sincere sympathy at this (very) difficult(, sad) time. My thoughts and prayers are / will be with you.

I was going to write that you could also say '(sudden) deaths,' but then I had second thoughts about the plural; perhaps just 'death' might be better, but see what others say. 'Loss' is euphemistic, but at least it avoids that issue.

*f5* Too late -- #1 and #2 are of course also fine.

Danke an alle