I have a 19 year-old roof with 2" x 10" roof rafters that are not of good quality wood; they were originally sistered on one side of some of the existing raftes with 2" x 10" boards but were apparently not fastened together properly (lag bolts) with not enough fasteners. The pitch of the roof is relatively shallow; it is not a steeply pitched roof.
What is the best way to insure a permanent fix considering the dimensions of the existing rafters and the amount of downward stress at the ridge?

Downward loading on roof rafters should transfer to the outside wall. 2X10 is a pretty substantial rafter and I don't know why they would have sistered unless the existing rafter is cracked. Do you have collar ties across the rafters?

Glued and screwed is the best mthod of sistering a rafter. How many roofs are on the bulding? One of the areas I have worked allows up to 3 roofs but that does cause the rafters to start bowing in some cases