If your financial house is in order, than you are free to spend your money on whatever it is that brings you joy. Or to give it away however you see fit.
I have seen people who just can't seem to hold back from giving free advice - constantly! - and was so glad that those individuals were not members of my own family (where I wouldn't be able to escape their advice).
If by chance the money situation is tight for your brother and sister-in-law right now - even if they're not talking about it - that could cause your SIL to view your purchases through the lens of her own difficulty.
I would say that if money is tight for you, it's probably best not to ever mention that in front of your SIL.