Have they told their mortgage broker the money from you will be a loan? Most lenders demand to know what other loans the borrowers have outstanding.
I would give them money if you want, and then back away. I would not have wanted my parents or in-laws involved in any decisions around buying a house.
I bought a house shortly after DH and I started dating. When we got married, I quit-claimed the house into both our names. Did not once occur to me to ask my parents or his what they thought of that.