Multi Mortgage Signers

[Notmyfield]

Ontario
My Wife and I are renting a house and the owner is selling,so we have to move.
Daughter and Son-In-Law own one home and went to the bank to see about another mortgage as an investment property and were going to rent that to my Wife and I...but their down payment was going to be to much.So they were told at another bank about a "Mortgage Family Plan" The four of us went to that bank and gave all our incomes and debts...and we were pre-approved for a Mortgage.Daughter an S-I-L will be providing the down payment.Son-In-Law wants to eventually make this a buisness for our Daughter and himself and have my Wife and I sign off on the Mortgage aggreement as soon as possible after the Mortgage papers are signed.
Wife and I were going to rent a house...but if all four of us are signing the Mortgage papers...are we not all equal in the future property?We (wife and I) do not have the down payment or money for any repairs,where they do.There is also a "5% cashback" which they beleive they will get.Would the cash back not be equal?
I was thinking... why pay them rent when we could be paying for the mortgage.Paying for the Mortgage would at least be sharing with the ownership of home.It's not like my wife and I wouldn't be participating in this venture.If we just rent and I die,my wife still has to rely on her own income for rent...But if we shared the Mortgage and I died,at least the Mortgage Insurance would relieve my Wife of any financialhardship.
I am retired due to a disability and receiving pensions.
I really feel that my Wife and I can share this future home with Daughter and Son-In-Law.They would both get it all when my Wife and I are deceased anyway.
I do understand that they want to invest for the future...and we are proud of them for that...but need to know the best advise for my Wife and I.
Will we be equal partners in his house purchase legally...and what about the "cash back"...should it not be equal as well?
Any and all advice would be greatly appreciated.Thanks...Notmyfield