OP here. Thanks for all the responses. To the pp that asked why we gave MIL the pieces in the first place - it was done while I wasn't home
To the pp that said matching pieces aren't "in" anymore. Yes, they aren't matchy-matchy but the loveseat and coffee-table and one end table (which is what's left) is odd and even finding LR chairs that fit in (not even match, but fit in) is hard.
We really can't afford to buy new furniture, but I will if I must - like others mentioned, there's always credit.
Thanks everyone for at least validating what is a pretty weird dynamic with the inlaws. We all have a pleasant enough relationship that I don't want to appear to be the bitch demanding used furniture back. But I am kind of mad since SIL is pretty well off (far richer than we are) and I'm feeling sorry for myself that we have to go out and buy new furniture for our own living space (that we use) just so SIL can have a furnished in-law suite that's rarely used. But that's just me feeling sorry for myself.