Doctors, dentists, and yes, your mother-in-law all give different advise about transitioning off a bottle.
Here are a few general guidelines.
Introduce an open cup at 6-months of age.
Only put a little liquid in the cup and tilt the cup up (not the child’s head) so that the liquid slowly enters the child’s mouth. You are aiming for your child to take ONE sip by closing their lips. Around 10-month of age, you can introduce a straw cup. Most children will have no clue how to suck through a straw so here’s a tip. The adult holds a straw and pippets (put straw in the liquid and occlude the top of the straw with your finger so that there is a small amount of liquid in the straw) a SMALL amount of liquid. Place in your child’s mouth and when they close their lips around the straw, release your finger so that the liquid goes into their mouth.
Shortly, your child should start closing their lips around the straw and sucking while you release the liquid. Proceed to a straw cup once your child is sucking. Most doctors suggest that a child starts to transition off a bottle at 12-months of age.
One tip on transitioning is to do it slowly. Start with a mid-day bottle instead of the first/last bottle of the day. If you are giving 4 bottles a day, say 7am, 11am, 3pm, 7pm, start with an 11am or 3pm bottle first. Replace it with a straw cup. After a week, replace the other daytime bottle. After a week, transition the 7am bottle, and then a week later the 7pm bottle. Some kids can transition all of their bottles in one week, but most will not do it so quickly. There is some controversy on the topic of sippy (spout top) cups. Some moms love them and won’t give them up. I didn’t use them for my son and I find that most Speech Pathologists do not recommend their use. I’ll let you make your own decision, but know that an open cup and a straw cup allow the child to develop more mature oral patterns.