I am trying to help my mother fix her kitchen sink (her house was built in the 1950s). It seems to back up occasionally and she spoke to a plumber who recommended that she replace her 1.25'' pipes with 1.5'' pipes and add a Studer vent.
1) Do most new sinks use 1.5'' pipes by default?
2) Isn't there some sort of easy pipe converter that will fit on the 1.25'' sink drain and allow me to use 1.5'' pipes the rest of the way?
3) I am not clear on what a Studer vent would do here, although I don't see any sort of venting going on under the sink with this drain as it is currently (I also looked under the house today and didn't identify any sort of venting system). Can you clarify as to what a Studer vent would do here and where should it go (underneath the sink? Under the house?)
4) Is this a reasonable change to be undertaken by someone with novice/intermediate plumbing skills?

Thanks in advance.

Bathroom sinks have 1 1/4 while kitchen should be 1 1/2 as well as a tub, shower gets 2". The drain hould be 1 1/2 all the way including trap and drain assembly. If he wants to add a AAV (studor) he thinks the vent is clogged and for some reason does not want to rod the vent from the roof. I think he is fishing and doesn't have a clue. You can replace the drain pipes if you have some small amount of skill and can read and follow instructions. Installing Kitchen Sink Drain Plumbing With S-Trap and Slip-Joint Fittings
Note; "S" traps are outlawed in all Plumbing Codes

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1) Do most new sinks use 1.5'' pipes by default?

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Most of our new installations still use 1 1/2" drains but increase to 2" going into the wall. The size was increased when disposals became popular and garbage was introduced in the system.

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2) Isn't there some sort of easy pipe converter that will fit on the 1.25'' sink drain and allow me to use 1.5'' pipes the rest of the way?

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You can increase the size of the drain lines only if you replace them from the trap to the vent stack. This would consist of removing the cabinet and opening up the wall to replace the drain line.

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3) I am not clear on what a Studer vent would do here, although I don't see any sort of venting going on under the sink with this drain as it is currently (I also looked under the house today and didn't identify any sort of venting system). Can you clarify as to what a Studer vent would do here and where should it go (underneath the sink? Under the house?)

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Look on the roof above the kitchen. If you can see a pipe exiting the roof line then the sinks's vented. If you don't see a pipe get back and we'll discuss Studor Vents.

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4) Is this a reasonable change to be undertaken by someone with novice/intermediate plumbing skills?

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If you're referring to a Studor Vent the answer's, Yes! A Studor Vent will install just downstream from the trap,(see image) in the cabinet.
Good luck, tom