Another - Acquiring parents house question

Objective-

A and B acquire A's parents house ( parents C and D) at lowest cost possible. C and D find another suitable house within 3-6 months to purchase as primary residence. A/B Looking to buy C/D between April-July of 2027. Then allow C/D to live in the basement for up to 6 months as to look for a new residence.

Living Trust? – If we go this route, can B and A live in this house as their primary residence even though C and D will live elsewhere?

If not living trust what is the most efficient way to purchase the house. Want to make sure C and D have enough liquid funds to purchase a new home.

Assets-

A and B- House in IL with rough market estimate at 370K.

Still owe- 165k

Currently financed at 3.875%

C and D- House in IL with rough market estimate of 435k

Still Owe-167k

Currently Financed at 2.99%

To be transparent, I honestly dont know how a trust would work. The reason I am interested in the trust would be to avoid the realtor fees of A/B buying and C/D selling. Also possibly capturing their existing mortgage rate. Buying now A,B,C and D would get hit with realtor fees on the sell and buy.