My husband and I are purchasing a duplex which my sister and her husband will be sharing with us. I would like to put them on the title. Can this be done and what are any of the negative affects. They also gave us half the down payment so they are invested in the property just not on the loan.

Have you closed on the purchase transaction? If you have, it may be too late because depending on the wording of your mortgage, adding them to the title may need to be approved by the mortgagee, which may require a refinance arrangement ("re-fi"). And even it permitted by your mortgage, your plan has the obvious flaw that, if your sister and brother-in-law fail to make their share of the payments, you will still be obligated to make them.

If you haven't closed, then add them as co borrowers to the mortgage so they will be added to the title at closing.

As noted, if you have already closed on the loan, you may be required to do a "refi" to change and add them to the loan. ( unless they are already on the loan papers)
It will be up to the lender if they will allow someone to be added.
Are they listed on the loan papers also?